Recently, numerous studies have been done on the micro- and nano-scale equipment because of their importance and wide range of application. Micromixers are among the equipment in which two or more fluids are mixed and have applications in the processes, such as chemical synthesis. In this research, a numerical investigation using finite volume approach is done on mixing two incompressible fluids in 3D mixers with T- and double-T-(DT) shaped geometries in the range of Reynolds numbers 75-400. One of the important parameters for the quantitative analysis of the mixing performance of micromixers is the mixing index. So, the effects of different geometries, Reynolds number and channel length on this parameter are studied. The results show that, at different Reynolds numbers, the mixing index of fluids in the DT-shaped channel with 90 degrees is less than the corresponding one in T-shaped mixers because changing the flow regime occurs at higher Reynolds numbers in the DT-shaped channels. The amount of mixing index increases by decreasing the angle of branches in the DT-shaped channel. It is observed that the mixing index of fluids increases along the channel, which tends to a constant value far away from the inlet.