BACKGROUND: Labor pain relief has been considered since many years ago. Heat as a non pharmacological method of pain relief helps reducing the pain intensity and increases the pain consistency. The aim of the study was to determine the effect of the heat therapy on the labor pain in primigravida women. METHODS: In this clinical trial study, 64 low risk nulliparous women were randomly divided into two heat therapy and routine care groups. In addition to the routine cares, warm bag were used for the heat therapy group for the low back, from cervix dilatation of 3-4 cm to the end of the labor's first stage and for perinea at the second stage. The pain intensity was determined by McGill pain questionnaire in dilatation of 3-4, 6-7 and 9-10 cm and at the end of the labor's second stage. Data was analyzed using t-test and chi square test by using SPSS 11. RESULTS: Results of research showed a significant decrease in the pain intensity in the heat therapy group at the first stage and the second stage of labor and comparing two groups showed significant difference(p < 0.001). CONCLUSIONS: According to the results of this study, it seems that heat therapy in addition to its beneficial effects, causes the mother to sense the labor pain in a lower pain severity.