The aim of this research is to study the social mobility of individuals in Thai society during the Early Ratanakosin Period, that is, from the reign of King Rama I to the reign of King Rama III (1786-1851), with emphasis on social mobility within the system of government services, the means by which most individuals became socially mobile, In addition to the Introduction, which presents basic sociological concepts concerning social mobility, the study comprises five chapters. Chapter One deals with the social stratification of Thai society, relation ships between the classes, and basic concepts concerning social mobility during the Early Ratanakosin Period. Chapter Two concerns the socio-political conditions the affected the circumstances of social mobility. The Introduction and the first two chapters provide a basis and guidelines for studying social mobility in the various socio-political circumstances which in turn provide the criteria and scope for the study undertaken in this research. Theses circumstances include the ascendency of a new king to throne, normal socio-political circumstances (times of peace), and abnormal socio-political circumstances (time of war) Chapters 3, 4 and 5 respectively deal with the categories of individuals who became socially mobile in each of the circumstances, the causes and factors affecting social mobility, the types and characteristics of social mobility as well as the effects on Thai society as a whole. Form the findings of this study the social stratification of Thai society in the Early Ratanakosin Period shows a division into two large classes, the ruling class comprised of princes and nobles and the ruled comprised of commoners (phrai) and slaves. In addition, there were other groups of individuals with special characteristics differentiating them from the former to classes. These latter groups included individuals outside the phrai system, namely, Chinese and Westerners on one category and members of religious orders in another. It appears that princes and nobleshad high rates of social mobility in all circumstances; likewise, priests experienced social mobility in all circumstances although in smaller numbers than princes and nobles. Commoners, as well as Chinese and Westerners, who were outside the phrai system, seem to have clearly achieved social mobility only in peace time. As for slaves there appear to have been no individual cases of social mobility even though laws provided opportunities for such mobility. In addition, the prices and nobles of tributary townships had high rates of social mobility in times of war, which arose from external political conflicts. Finally, women seem to have achieved some social mobility but less than that of men. Social mobility consisted of both upward mobility-either a normal rise in rank and position or a cross-rank rise-and downward mobility-removal of one's titular rank and removal from one's government position to the status of commoner, imprisonment or execution. In either case, the effects were felt by the individual'sfamily and relatives. It appears that most social mobility was vertical and that most there was more upward mobility than downward mobility. In addition, a number of individuals, nobles in particular, managed to both rise and lower their status several times within their lifetime. Comparison of the social mobility of these nobles within their lifetime with that of their fathers shows that their upward mobility was equivalent to that their fathers. Upward and downward social mobility were dependent on various causes and factors, both external and internal. It can be concluded from this study that the aforementioned features of social mobility resulted in Thai society being an open society. The various groups and categories of individuals-the ruling phrai system members of religious orders, as well as women-all had opportunities for social mobility even of the opportunities of the ruling class were greater.