Abstract: The relationships between Yickers hardness (H) and Temperature (T) for six industry alloying tool steels and seven WC-Co sintered alloys have been investigated. When T is lower than tempered, the reversible softening value for tool steels undergone quenchingtempering secondary hardening treatment is a linear function of T. The diferences among reversible softening factors for various high speed steels (HSS) are very small. Hence, a simple relation to estimate the hardness of HSS at T below 500℃ is presented. When T is higher than tempered, the H-T relation of tool steels is complicated, but H is still mainly determined by the room temperature hardness after heat treatment. For WC-Co sintered alloys, the H-T relation is linear up to 800℃, and the relation between H and cobalt content (Co%) is also linear when Co<20%. The relation tor estimating hardness of WC-Co alloys from T (<800℃) and Co (3%-20%) is presented.