Argemone mexicana methyl ester was prepared by a two-step transesterification process due to its high acid value, and was tested for its combustion analysis with variation of blends at a fixed compression ratio of 17.5 and at a fixed load of 8 kg. B40 gives higher combustion pressure than that of standard diesel due to shorter ignition delay. The mass fraction burnt of blends is slightly higher than that of standard diesel. At a particular load, the heat release rate increases with lower percentage of biodiesel, while the cumulative heat release increases with increases in proportion of biodiesel in the blend.