In this work, the effects of liquid viscosity on flow patterns, gas slug velocity and gas slug length in a horizontal circular microchannel with an inner diameter of 530 μm have been investigated by using a high speed camera. As the working fluids, nitrogen gas for the gas and aqueous solutions of glycerol with high viscosities the concentrations up to 78.0 wt% for the liquid were used. Kinematic viscosities and surface tensions of liquids were respectively ranging from twice to 35 times and from 0.9 to 1.0 than that of distilled water. The superficial gas velocities, jG, were ranged from 0.01 to 13.0 m/s, and the superficial liquid velocities, jL, were from 0.05 to 0.30 m/s. The flow patterns observed were slug flow and disturbed ring film flow. The velocity of gas slug for the system of nitrogen-distilled water two-phase flow coincided well with that in case of homogeneous flow, on the other hand, velocities in the case of glycerol solution with the higher viscosities showed the higher values than equation of homogenous flow and equation proposed by Hughmark. The correlation of the distribution parameters, C0, of two-phase flow were proposed in the present paper as a function of the ratio of kinematic viscosity of the liquid to that in distilled water. The tendency was observed that the lengths of gas slug became short when the viscosities of the liquid were increased.