In this work, the group contribution method of Chickos et al. was applied to estimate the fusion enthalpy of isonicotinic acid, and the obtained result(29.2 k J·mol~(-1)) showed a large difference with the value(135 k J·mol~(-1)) as reported from literatures and as determined by differential scanning calorimetry(DSC). The results of DSC/TG measurement showed that the phase transition of isonicotinic acid from 187.27 °C to277.47 °C underwent a sublimation process, with a sublimation enthalpy of 128.03 k J·mol~(-1). An efficient analytical technique combining pyrolysis and gas chromatography/mass spectrometry(Py-GC/MS) was used to prove this conclusion. In this work, the group contribution method of Chickos et al. Was applied to estimate the fusion enthalpy of isonicotinic acid, and the obtained result (29.2 kJ · mol -1) showed a large difference with the value (135 k J · mol ~ (-1)) as reported from literatures and as determined by differential scanning calorimetry (DSC). The results of DSC / TG ​​measurement showed that the phase transition of isonicotinic acid from 187.27 ° C to 277.47 ° C underwent a An efficient analytical technique combining pyrolysis and gas chromatography / mass spectrometry (Py-GC / MS) was used to prove this conclusion.