A method of forming an image using a photothermographic material containing a support having thereon an image forming layer which contains an organic silver salt, silver halide grains, a binder and a reducing agent, the method including the steps of: imagewise exposing the photothermographic material to light to form a latent image; and simultaneously or sequentially heating the exposed photothermographic material to develop the latent image, wherein at least two matting agents are contained on one surface of the support, and an average particle size LA of Matting agent A and an average particle size LB of Matting agent B satisfy the following relationship: 1.5.ltoreq.LB/LA.ltoreq.6.0, provided that Matting agent A is the matting agent having a largest weight ratio; and Matting agent B is the matting agent having a second largest weight ratio.