This study was conducted to propose an accurate measurement method of charred thickness in structural glued laminated timber (glulam) exposed to standardized fire temperature conditions for evaluation of the fire resistance of Japanese larch (Larix kaempferi) structural glulam. The compressive strength of the test piece collected from the structural glulam exposed to the standardized fire temperature for two hours was analyzed. The results showed that there was residual strength of 14% in the boundary area between sound and charred part. Therefore, the thickness of charred part should be calculated as approximately 4.5 mm less than what is measured visually. The cell wall thickness between sound-, boundary-, and charred-part exposed to high temperature was observed under a microscope. Higher temperature resulted in a thinner tracheid cell wall, and the radial tissue was broken down. The transformation of the boundary part of these microstructures progressed gradually until full carbonization of wood occurred. Based on compressive strength tests and microscopic observation in the boundary part of specimens exposed to high temperature, the current visual assessment of charred thickness measurement is safe in terms of structural aspect but not efficient in terms of wood utilization point of view due to exception of load carrying area of the member.