A numerical method is proposed to measure viscosity relative to a reference fluid, where the profile of velocity is simulated, and to analyze the relationship between the volume flow rate ratio, the absolute viscosity ratio, and the interface location under two co-flowing streams in a micro-fabricated T-channel. Neither absolute flow rates nor pressure difference are required, all the values between the volume flow rate ratio, the absolute viscosity ratio, and the interface location are obtained easily. By using these data, the absolute viscosity of the fluid in Micro-Fluidic Viscometers is obtained.