Flow observations through a forward-facing step channel were conducted to complete the mechanism for instability of planar entry flow of rather dilute polymer solutions, and the types of unstable flow patterns as well as the mechanism for the back-and-forth movement of the Goertler vortices have been studied in detail. Our observations have confirmed that four types of flow patterns exist in the unstable flow regime, where the Goertler vortices occur around the salient corner, and the manner of instability becomes more complex as the solution concentration increases. Furthermore, the appearance and coalescence of the Goertler vortex pairs cause their back-and-forth movement along the longitudinal direction of the channel, and three-dimensional lip vortices play an important role in the appearance of a pair of the Goertler vortices.