We studied an optical interaction on an optical window and blood boundary during the CW laser irradiation in 660 nm band until blood charring occurrence. We previously reported that a pre-charring optical behavior may be detected by diffuse-reflected-light power time-history. The aim of this study is to measure absorption coefficient (μ a) and reduced scattering coefficient (μ' s) of a blood model to explain this pre-charring optical behavior. The blood model sandwiched between 2 glass slides to simulate the interface between blood and the optical window was used. A double integrating sphere system was constructed. The red laser in 660 nm band was irradiated to the sandwiched blood model. Fourty W/cm 2 in irradiance was used as the maximum irradiance during irradiation via the laser catheter in vivo. μ a and μ' s in the irradiated laser wavelength were measured continuously until blood charring occurrence using inverse adding doubling analysis. Continuous μ a increase of 5-10% from the initial value until charring was observed. Decrease of μ' s with 8-10% during 15-30 s before charring following broad peak was obtained. We think these μ a and μ' s changes may explain the pre-charring optical behavior detected by the diffuse-reflected-light power time-history in our reported study.