<p>This study proposed a method to quantify the unique patterns of longitudinal residual stress in large-diameter Japanese zelkova (<i>Zelkova serrata</i>) logs and evaluated their characteristics using 11 logs from 8 trees. While the lateral distributions of longitudinal residual stress show the smooth bell curve patterns in many other species, those of Japanese zelkova logs often exhibit a unique and varied zigzag pattern, as if they had “spikes” along their lateral distribution. The study defined an index called “spike level” to quantify the intensity of spike-shaped strains that deviate from the expected trend of the lateral distribution. The following key findings were obtained through the quantification procedure and investigation of the spike level: 1) Each residual stress distribution was composed of two components: a global distribution that made a smoother distribution, and local stresses that made spikes on the distribution. 2) The index “spike level” varied widely among the samples and reflected well the deviation from the smooth bell curve pattern. 3) The residual stress distribution showed three-dimensional complexity and was not axially symmetric around the pith. This quantification method enables comparison between logs and provides a foundation for further research on the unique residual stress patterns in Japanese zelkova, including the mechanisms of its formation and its impacts on wood processing.</p>