Photothermographic materials that contain silver halide grain and organic silver salts are widely used in dry processing imaging systems. Because its characteristic curves are very sensitive to development condition, precise calculations of the characteristic curves under various development conditions are essential for practical development system designing. Though some previous models for the characteristic curves had been proposed, those uses are limited because they based on some ideal assumptions. In this study, change of experimental characteristic curves under various development conditions was analyzed, and a new model using semiempirical simulation method was found. This method gives both the fraction of silver halide grains receiving a latent image exposure and the radius of sphere of influence from the experimental characteristic curves of several development conditions. The fraction is an intrinsic value of the film, and it represents the photosensitive character including fog. The radius of sphere of influence represents the degree of development, and its rate of increase is constant under fixed development temperature. These parameters are useful for analysis of development rate. Once these parameters were defined, exact characteristic curves of any development conditions are calculable. The found simulation method is useful for analysis and design of practical photothermographic system.