The hot hardness was measured on the (010) plane of primary cementite in unidirectionally solidified iron-carbon, iron-carbon-chromium, and iron-carbon-boron alloys at temperatures up to 973 K, using a hot hardness tester equipped with an indenterheating system. The hardness of paramagnetic cementite against temperature was represented by the Ito-Shishokin relation. On the other hand, the hardness of ferromagnetic cementite deviated to low values from the Ito-Shishokin relation found for paramagnetic cementite. This deviation occurred from magnetostriction, because the thermal softening coefficient of cementite was found to relate to the thermal expansion coefficient regardless of the magnetic state similar to that found with fcc and bcc metals. Chromium and boron increased the hot hardness of cementite.