Effects of liquid viscosity on coalescence of a pair of bubbles were experimentally studied by generating nitrogen gas bubbles in a pool filled with silicon oil. We focused on two types of coalescence/bouncing of bubbles. The first type was those between a rising bubble and a free surface, and the second type was those between of a pair of bubbles rising side by side. The former is considered as the extreme case, which is equivalent to coalescence/bouncing between bubbles with finite and infinite diameters. The later is more practical. That is, bubbles were generated simultaneously from a pair of pin-holes on a pipe horizontally placed. We studied the viscosity effects on coalescence of bubbles by investigating behavior of bubbles with both liquid viscosity and bubble diameter as the parameters. Previous studies reported that We number based on approach velocity could predict whether two bubbles would bounce or coalesce in super purified water. Our experimental results show that the threshold We number in the case of low viscosity liquid indicates same order as the one in the case of super purified water. On the other hand, in the case of large viscosity liquid, the threshold We number differs significantly from the one in the super purified water.