Properties of low beta quasi-perpendicular shocks for various ion to electron mass ratios are studied by using a one dimensional full particle simulation code. It is shown that shock reformation process changes depending on the mass ratio. In unrealistically low mass ratio runs, the reformation is due to accumulation of gyrating specularly reflected ions at the upstream edge of the foot. At the realistic mass ratio, on the other hand, the above process is strongly modified by ion thermalization associated with the modified two-stream instability in the foot. At sufficiently low mass ratio the Buneman instability gets excited in the foot while it is stabilized at higher mass ratio. The two reformation mechanisms are compared in detail by additional runs.