The present paper deals with the theory of laminar boundary layer on a nonaxisymmetric conical body at supersonic flight with special reference to the effect of gas injection on the body surface. The Moore's method of solving boundary layer flow on a supersoic circular cone at small angles of attack is extended to the case of a conical body with nearly circular but otherwise arbitrary cross-section and further to the case of boundary layer with gas injection. The outer edge conditions are determined by the inviscid conical flow solution around the body obtained by an extension of the Stone's method for a circular cone. It is found that an exact similar solution to the first order perturbation equations of the boundary layer with gas injection on the conical body as stated above exists at a specific distribution of the rate of the gas injection. In case of air injection into air stream over isothermal conical body surface similarity of the boundary layer holds at the injected mass flux rate inversely proportional to the square root of the meridional distance from the body apex. The effects of mass injection on boundary layer characteristics are discussed for the cases of several shapes of conical bodies at M_∞ = 5.0 and Prandtl No. P_r = 0.738. The results show that the injection effects on heat transfer and skin friction are more significant at the leeward side than at the windward side in the cross flow plane.