In this work we studied the effects of heat treatment for stress relief on the microstructure and mechanical properties of steel API 5L PSL2 X65MS, manufactured by UOE process, with plate produced by the controlled rolling process. The samples were subjected to heat treatment at three different temperatures (550 degrees C, 600 degrees C e 650 degrees C) for 1,5 hour in muffle furnace, and cooled to room temperature in still air. After the heat treatment the material was subjected to tensile, hardness and microhardness mechanical tests, and microstructure analysis by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average grain size of the samples in the as-received condition was 3.3 um. The heat treated samples presented no significant change in the average grain size, which were 3.5 μm. But, there was a downward trend in the hardness and yield strength average values, when compared to the results of the as-received samples. The heat treated samples at temperatures of 550 degrees C and 600 degrees C presented no significant changes in the tensile strength limit average stress. However, the tensile strength limit average stress value for heat treated samples at 650 degrees C presented a downward trend. Nonetheless, the downward trend in the hardness, yield strength and tensile strength limit average values for the heat treated samples are within the standard specified values.