Thai pepper is normally obtained from two cultivars, the Sarawak and Sri Lankan which are grown locally in Thailand. This study aims to evaluated the quality of black pepper from two cultivars grown at Chantaburi, the major site of pepper plantation in Thailand by determination of pepper oil and piperine contents. Quantitative analysis of total pepper oil and piperine contents showed that the Sri Lankan cultivar (2.73%, 4.96%) contained significant higher levels of the pepper oil and piperine (α=0.05) than the Sarawak cultivar (1.78%, 3.82%). In term of pepper oil composition, the analysis by GC and GC-MS revealed that both cultivars contained approximately 30 similar components. Among these, α-pinene, β-pinene, sabinese, ∆³-carene, limonene and β-caryophyllene appeared to be the major ones. For the minor components, these could be identified and grouped as seven monoterpene hydrocarbons, eleven sesquiterpene hydrocarbons and six oxygenated terpene compounds. From these results, it is concluded that the black pepper from the Sri Lankan cultivar contains better quantity and quality of its volatile oil and piperine than that from the Sarawak cultivar. In comparison with some black pepper products from other countries, the black pepper cultivated in Thailand (both Sri Lankan and Sarawak cultivars) shows more piperine but less volatile than those from India, Brazil and Malaysia. In the maturation of pepper berries, the volatile oil and piperine contents reached their maximum at 3 month old of pepper berries and thereafter decreased. However, the optimum age of the berries to be harvested for black pepper product is 5 month old. During 2 to 6 months of pepper berries maturity, there is no significant variation (α=0.05) of the volatile oil composition. After harvesting, the black pepper can be stored at room temperature at least eight months without affecting the volatile oil content and composition.