The approach to the experiment of the thermal analysis for the undergraduates in our university has been reported. The solid phase transition behavior of NH₄NO₃ was analyzed by the self-made DTA apparatus. The transition temperatures and heats of transition of three phase transitions were estimated from the DTA measurement. The data obtained by the students were compared to the reference data. The experimental average value of the first transition temperature was higher than the reference value by 8^oC, whereas those of the second and third ones agreed within 2^oC. The experimental average values of the heats of the transitions were lower than the reference values by 40%. We discussed some problems about the abnormal estimation of the heats, the validity of the reference sample for the correction of the heats, and the measurement techniques.