Experimental observations of the creep response of a commercial sintered silicon nitride ceramic are presented. The stable microstructure of this material at high temperature contains secondary crystalline phases which result from partial devitrification of the intergranular phase, The widths of amorphous films along grain boundaries (between silicon nitride grains) and phase boundaries (between silicon nitride and secondary phase grains) are characterized by transmission electron microscopy, The thickness distributions of grain-boundary films before and after creep are analyzed by a statistical method, While the film widths are highly uniform before creep, a bimodal distribution is observed after creep. The results suggest that viscous flow of the boundary amorphous films occurs during creep deformation.