Biodiesel production by transesterification process employing alkaline catalyst has been paid to a lot of attention due to its high conversion rate. In this work, sodium hydroxide (NaOH) catalyzed transesterification of rapeseed oil with methanol was carried out in a bench scale experimental reactor. The effects of molar ratio of methanol to rapeseed oil (4︰1-8︰1), catalyst concentration (0.5%-2%), reaction temperature (30-70℃) and reaction time (30- 150 min) on the conversion rate of transesterification were investigated. Composition of biodiesel was determined by using GC/MS analysis. Results showed that the suitable parameters for reactions were as follows: temperature 50-60℃, molar ratio of methanol to rapeseed oil 6︰1, NaOH concentration 1% and reaction time 60 min with highest conversion rate of transesterification 96.7%. In addition, it was found that biodiesel was mainly consisted of 9-octadecenoic acid (z)-, methyl esters, 13-docosenoic acid (z)-, methyl esters, 9, 12-octadecadienioc acid, methyl esters, 11-eicosenoic acid, methyl esters and 9, 12, 15-octadecatrienoic acid, methyl esters. Among which, the relative content of 9-octadecenoic acid (z)-, methyl esters was the highest one, reaching 50.30%.