The age-hardening behavior at around 750°C and microstructure analysis of experimentally prepared heat resisting steels based on SUH38 containing phosphorus in the range of 0 to 0.3 mass% have been investigated, in order to identify the formation mechanism of the homogeneous distribution of fine carbide within grain interiors (MDP: matrix dot precipitation) by the addition of phosphorus. The steel does not show age-hardening until the phosphorus content in the steel beyond 0.15 mass%, and the hardness increases from 200 HV to 350 HV with increasing phosphorus content. The high age-hardening was caused by the high density of fine M23C6 carbides with cube on cube orientation relationship between γ and M23C6. Phosphorus was found to exist in γ-matrix as substitutional manner after solution and aging treatments. The fine carbide formation mechanisms within grain interiors are attributed to an interaction between phosphorus and carbon in solution.