[en] A transient two phase flow was investigated in an air-water two phase system. Transient experiments were carried out by increasing liquid flow rate with time to change flow patterns from slug or plug to bubble flows. Void fraction measured in the transient conditions is a little less than the one in the steady state which is evaluated by inlet flow rates of air and water at each time. This small difference is due to a limited effect of gas compressibility in the flow patterns in which the liquid is continuous and the gas discreet phases. An increasing rate of liquid flow rate has a negligible effect on the void fraction transient. In low gas flow rates a pressure drop in transient conditions is a little larger than the one in the steady state and the increasing rate has no effect on the pressure drop. On the other hand, in high gas flow rates the transient pressure drop is less. This tendency is consistent with the void fraction transient. In this case the increasing rate significantly affects the pressure drop and its higher value makes a lower pressure drop. (author)