The thermal history before the final heat treatment (hardening) of high speed tool steels affect the microstructure and, consequently the final properties of the tools. The influence of quenching temperature on the mean austenite grain size and size distribution and volume fraction of the primary carbides, MC and M(6)C, in AISI T15 samples prepared by hot isostatic pressing and liquid phase vacuums sintering was quantitatively investigated. The quenching temperatures were 1135, 1160, 1185, 1210 and 1235 degreesC. The mean austenitic grain size as well as the distribution and the volume fraction of the MC and M(6)C primary carbides have been determined by analyses of the scanning electron micrographs using the Quantikov quantitative method. Specimens prepared either by hot isostatic pressing or by vacuum sintering show no modification in the mean grain size for increasing quenching temperature. Similar behavior was found for the measured carbide mean size. The mean size and the volume fraction of the MC and M(6)C carbides were similar for the hot isostatic pressed specimens. For the vacuum sintered specimens, on the other hand, the MC carbides present larger mean size and volume fraction, besides a larger fraction of coarse carbides (1,5-3mum). The main results show that the quenching temperature increase of the AISI T15 steel does not influence both the mean grain and carbide sizes.