For fabricating the microfluidic chips with uniform sized microchannels, the standard deviation of upper width and depth of trapezoidal microchannels was proposed to characterize the size uniformity of microchannels. The influence of injection molding processing parameters on the size uniformity of microchannels in cyclic-olefin-copolymers(COC) microfluidic chips was studied, providing guidance for the setting of processing parameters. The experimental results show that the processing parameters have greater influence on the upper width of microchannels than on the depth. The maximum range of the standard deviation of the upper width is 3.82 μm and that for the depth is merely 0.61μm. In addition, the influence of mold temperature on the size uniformity of microchannels is the most significant. As the mold temperature increases from 80 ℃ to 120 ℃, the size uniformity of microchannels witnesses a slight decrease, and then increases gradually. The lowest standard deviation of the upper width is 3.24 μm, and the decreased percent is 54%. The holding pressure has the least impact on the size uniformity of microchannels. The size uniformity of microchannels increases gradually with the pressure within 80~120 MPa, but the maximum standard deviation is only 1.29 μm.