The microstructural and mechanical properties change that occur during Postweld Heat treatment in 24 mm wall thickness HSLA steel- API X70 pipe have been investigated by means of optical microscopy, micro-hardness, tensile and Charpy impact tests. The welding of the API X70 used in this work (0,13% C, 1,3% Mn, 0,14% Cu, 0,14% Mo, 0.04% Nb) was done by GTAW and SMAW processes, using different filler metal (ER70 S-6 and E8018-G). The postweld temperatures and holding time used were 550°C, 580°C, 610°C, 640°C and 640°C for an holding time of 60 minutes. The need for PWHT is driven by code and application requirements. In general, when PWHT is required, the goal is to increase the resistance to brittle fracture and relaxing residual stresses. Two postweld heat treatments temperatures, 550°C and 640°C, were found to produce a good combination of strength, ductility and toughness: at 550°C, (σ y=480 Mpa, A=31%, CVNzat=122 Joule, CVNfz=136 Joule) and at 640° C (σ y=472 Mpa, A=35%, CVNzat=134 J, CVNfz=154 J). However, postweld heat treatment between 580° C and 610° C led to a significant decrease in toughness due to the sideplate morphology of the ferrite phase and to the precipitation of the NbC at 610°C. The various microstructures that occur in the heat affected zone (HAZ) and the fusion zone (FZ) are described and the variables determining their formation are discussed.