The present invention provides a martensitic heat resistant alloy having a composition (A) comprising, % by weight: 0.03 to 0.15% of C; 0.01 to 0.9% of Si: 0.01 to 1.5% of Mn; 8.0 to 13.0% of Cr; 0.0005 to 0.015% of Al; no more than 2.0% of Mo; no more than 4.0% of W; 0.05 to 0.5% of V; 0.01 to 0.2% of Nb; 0.1 to 5.0% of Co; 0.008 to 0.03% of B; less than 0.005% of N: and Fe and inevitable impurities as the remainder, wherein (B) the contents (% by weight) of Mo, W, B and N satisfy the following formulae (1) and (2). B-0.772N>0.007 (1) W+1.916Mo-16.99B>2.0 (2)The martensitic heat resistant alloy of the present invention has excellent oxidation resistance, hot workability and ductility and exhibits high creep rupture strength in a range of relatively long rupture time at a high temperature.