Abstract: Investigations were made on the effects of Ms temperature and rate of cooling as major factors and alloying as a minor factor upon the extent of autotempering by using five low carbon steels. TEM obvervations were made on the precipitation of carbides from the low carbon martensite matrix, their shape, size and distribution being determined and crystal structure identified by selected area electron diffraction. Tensile, impact and ductile fracture toughness tests were made on specimens in autotempered, naturally aged and 200℃ tempered conditions. Basing upon the obtained experimental results a discussion is presented concerning the possibility of utilizing auto-tempering effect to heat treatment practice pointing out the value in energy conservation if the conventional tempering is substituted by sufficient au-to-tempering.