The pyrolysis of pure and crude glycerol from a transesterification process of waste cooking oil to bio-diesel was investigated using thermogravimetric analysis. Unlike the pure glycerol that shows a single mass drop, there were five distinct phases in the pyrolysis of crude glycerol. Pure glycerol decomposed completely within the temperature range from (133-158) to (240-279)degrees C depending on the heating rate. Leaving behind a small mass fraction (less than 1%) of pyrolysis residues. However, the crude glycerol is more thermally stable over the second and third decomposition phases range from (138-151) to (357-401)degrees C leaving a large mass fraction of residue (similar to 12-13%). Model fitting methods of the Direct Differential and Coats-Redfern methods were used to evaluate non-isothermal kinetic parameters in the second phase using 24 reaction models. The activation energy obtained by compensation effect of direct differential method and Coats-Redfern method were (82.611 kJ/mol and 74.890 kJ/mol) and (74.253 kJ/mol and 58.281 kJ/mol) for the pure and crude glycerol, respectively. The best reaction models obtained by the highest R-2 and the master plot. The reaction order was calculated by Avrami theory, and the average value of 0.918 and 1.317 was obtained for the pure and crude glycerol, respectively. (c) 2019 Elsevier Ltd. All rights reserved.