Transition of flow past a pair of side-by-side elliptic cylinders with aspect ratio A=2 is investigated by numerical simulations and the bifurcation analysis of the numerical results. It is already known that flows past a pair of elliptical cylinders are influenced by the gap ratio G and Reynolds number Re. Various flow patterns behind the ellipses have been found and the transition diagram from steady to semi-single/twin symmetric/deflected vortex shedding flows have been reported in our previous study (Peng et. al.1). This study focus on transition of in-phase/anti-phase vortex shedding synchronized forms. It is found that the in-phase and anti-phase vortex shedding flows can co-exist at 2.26≦G≦2.58. Hysteresis loops with increasing/decreasing G at constant Reynolds number Re=100 and with increasing/decreasing Re at constant gap ratio G=2.4 are reported, respectively. The hysteresis phenomenon of in-phase/anti-phase vortex shedding synchronized forms is also investigated in flows past a pair of circular cylinders. Keywords:anti-phase/in-phase