The major purpose of this research was to determine the Thai university environment factors affecting undergraduates' academic achievement. A sample consisted of 685 undergraduate students from five public universities : Chulalongkorn, Mahidol, Thammasat, Kasetsart and Silpakorn University. The University Environmental Inventory with 88 items was used to collect the data. After the data were factor analyzed by means of Principal Axes Method, orthogonal factors were extracted and rotated by the Varimax Method. The significant factors were : Learning Climate, Instructor'ร Behaviors Related to Teaching, Participating in Extra-Curricular Activities, Facilities, Administration and student Discipline, Sport and Recreation, Peer Personality and Instructional Service.