•The width of ITZ of steam-cured concrete was analyzed by microhardness.•A novel analysis method was used to obtain the microhardness distribution.•The change in ITZ observed by microhardness indicates the heat damage in steam-cured concrete. In order to understand the influencing mechanisms of curing temperature on the performances of steam cured concrete further, the microhardness characteristics of the interfacial transition zone between cement paste and coarse aggregate under different curing temperatures were studied through microhardness measurements, backscattered electronic microscopy (BSEM) and mathematical statistical analysis. The experimental results indicate that the microhardness of the hardened cement paste presents a significant statistical distribution characteristic. That is to say, there are two microhardness characteristic values which reflect the hydrated products and unhydrated cement particles, respectively. The curing temperature has a significant effect on the microhardness of interfacial transition zone. In other words, the microhardness decreases, and the thickness range of interfacial transition zone increases with increasing the curing temperature. The relationship of the microhardness value of the interfacial transition zone to the distance from the aggregate surface conforms to the Peal function.