Biodiesel is a prominent alternative fuel made from triglycerides. Generally, transestrification of triglycerides with methanol is the traditional method applied to produce biodiesel. The controlled un-sequential pyrolysis, however proves to be more effective in converting poor quality vegetable oil, animal fats, even rotten wasted cook oil into biodiesel. Because of complicated pyrolysis mechanism of triglycerides, thermodynamic calculation by computer simulation is applied to different pyrolysis reactions in order to study its pyrolysis pattern and control its pyrolysis behavior to obtain ideal product. Calculation results show that the cleavage of C-O bond takes place at the temperature of about 288degreesC while the beta bond scission (C=C bond as a bond) occurs at about 400degreesC. The distributions of pyrolysis products depend on the dynamics and kinetics control of different reactions. Maximum of diesel fraction can be obtained under appropriate reaction conditions.