Microstructure has a very strong influence on the sulphide stress cracking(SSC) resistance of high strength steels. In this work a commercial heat of a low alloy carbon steel was used. Six different heat treatments were performed using two austenitization temperatures (890° and 1110°C) and three tempering temperatures(500°, 600° and 700°C). The microstructure was analyzed using optical and electron microscopy(SEM and TEM). The SSC behavior was examined using slow strain rate testing (SSRT) of smooth samples. After quenchhg, packets of fine autotempered lath martensite were the main component, although retained austenite and plates of ferrite with very small carbides inside were also detected. After tempering at 500° and 600°C, the matrix did not change its morphology very much. The retained austenite was transformed into a thin film of carbides located between laths and in prior austenite grain boundaries (PAGB), The specimens tempered at 700°C showed recovery (more in the material austenitized 890°C) although most of the microstructure retained the laths morphology. In SSRT, materials tempered at 500° and 600°C broke during the elastic elongation and a mixture of intergranular and transganular fracture was observed. Materials tempered at 700°C broke in the plastic range and transgranular fracture we observed (although some interlath fracture parallel to the direction of deformation we also detected). Large carbides, together with inclusions are sites of crack nucleation and they may also help the propagation. High-angle boundaries (PAGB and boundaries of lath martensite paskets)are sites when cracks are arrested.