In experiments teaching, the results of solution enthalpies of KNO3 crystal measured by electric heating compensation measurememt are often not consistent with the literature value. Particle size of the crystal will affect measurement accuracy of solution enthalpies. Different grain size of the crystal are obtained by grinding and sieving, theirs solution enthalpies are measured respectively and integral solution heat are obtained. Differential solution enthalpies and integral dilute enthalpies are obtained by graphic method. The experimental results show that crystal size has greater impact on the measurement error of dissolution heat, and with the particle size of crystal decreasing the deviation from the literature values of integral solution heat, differential solution enthalpies and integral dilute enthalpies reduces. Therefore the crystal should fully grind in the experiments.