A computer program system THERMECH for THERmoMECchanical and THERmal-Hydraulic design of fusion blanket has been developed. The THERMECH executes blanket design by following processes: 1) preliminary thermal-hydraulic design of blanket by one-dimensional analysis, 2) pre-processing, calculation, modifying and post-processing of two-dimensional thermal-hydraulic and thermomechanical analyses, 3) pre-processing, calculation and post-processing of three-dimensional thermomechanical analysis, 4) calculation of inelastic strain and resultant stress by one-dimensional analysis. This paper presents the outline of the THERMECH program system and the application to JAERI Tokamak Power Reactor.