Abstract: Petroleum industry has important role in the process of industrialisation of Assam. There are four oil refineries in the state-Digboi Refinery, Guwahati Refinery, Bongaigaon Refinery and Numaligarh Refinery Limited. Bongaigaon Refinery, the third refinery in Assam was set up as public sector limited company in January 1972 and was commissioned in 1974 with headquarter at Dhaligaon in the old Bongaigaon district of Assam. The objective of the study is to analyse the production behaviour of Bongaigaon refinery using two production functions- Cobb-Douglas (C-D) production function and Constant Elasticity of Substitution (CES) production function.