Abstract This study evaluated the effects of torrefaction temperature on the grindability of Japanese cedar (Sugi) and fibrous biomass particles, while addressing the limitations of the conventional Rittinger's law. The standard Rittinger's law physically failed as the specific surface area difference () became negative at certain temperatures. To resolve this, a modified Rittinger's law utilizing the skeletal density difference (), which represents the intrinsic volumetric destruction, was proposed. This novel index consistently maintained a positive value, demonstrating its validity where conventional metrics fail. Analysis revealed species‐specific mechanical responses: cedar showed a peak in resistance at intermediate temperatures due to the interplay of brittleness and hardness, while fibrous biomass particles showed a continuous decrease in grindability. Scanning electron microscopic analysis further elucidated that these trends stem from distinct grinding mechanisms—surface stripping for cedar and volumetric grinding for fibrous biomass particles. These findings contribute to the global goal of carbon neutrality by optimizing biomass co‐firing and milling processes.