Process safety of hazardous phosgenation reactions during industrial chemical production is a critical issue. We herein have designed a compact flow reaction process for phosgenation reactions and successfully installed the system at a commercial plant. The hold-up of phosgene was significantly reduced and a leakage detecting system was incorporated in the commercial process. A scale-up method for determining the mixing condition of the commercial-scale flow reactor was the most important task in this study. The Villermaux-Dushman reaction was employed to evaluate the mixing performance of the T-shaped mixers with the mixer outlet diameter, D-m ranging from 0.5x10(-3) to 15.0x10(-3) m. The scale-up condition was determined by the absorbance of the Villermaux-Dushman reaction solution. Utilizing the experimental results, a diagram of the mixing performance in terms of Dm and the flow rate was obtained. It was found that the fluid segment size W-c (= D-m/2(0.5)) in a T-shaped mixer, and the Reynolds number of the mixer flow Re are important parameters for mixing in the T-shaped mixers. We derived the correlation between the mixing performance and W-c/Re-n. It was found that all of the mixers utilized in this study had the same correlation with an n value of 1.19. We also confirmed that the derived equation is applicable for scaling up the T-shaped mixers.