Three different polymers, a high density polyethylene (HDPE), a polypropylene(PP), and a polycarbonate(PC), were subjected to a flow visualization study in two rectangular channels with different width-to-height ratios, and the flow velocity profile in the channel was determined as a function of polymer melt temperature, channel wall temperature, and mass flow rate, as well as the pressure applied to the polymer melt in the channel and the geometry of the channel. The flow velocity data, which were measured by using a particle tracer technique and accumulated in a computer, were analyzed by a CAE program “POLYFLOW” developed on the basis of Bird-Carreau model to simulate the flow velocity curves in both vertical and horizontal longitudinal sections which involve the central axis of the channel. The extrapolation of the flow velocity curves to the inner wall surface allowed to estimate the contribution of slip flow on the channel wall (Vs) to the total flow velocity. When HDPE was subjected to the channel flow, the Vs increased with increasing mass flow rate, temperature of running polymer, and temperature of channel wall, but decreased with increasing pressure applied to the melt. More pronounced effects of the melt temperature and the mass flow rate was found on the wall slip phenomena for both PP and PC.