This research covers the experimental investigation of two phase flow instabilities in a horizontal in-tube flow boiling system whose test tube is relatively long. All experiments are conducted at constant heat input, system pressure and exit restriction. Steady state characteristics for five inlet temperatures are found and presented in pressure drop versus mass flow rate diagrams. Dynamic instabilities, such as pressure drop type, density wave type and thermal oscillations are found to occur for all the investigated temperatures, and the boundaries for the appearance of these oscillations are found. As a result, the stability boundary moves to lower mass flow rates with decreasing inlet temperature, which means that the system is less stable for higher inlet temperatures. It is found that the periods and amplitudes of the pressure drop type and density wave type of oscillations decreases with decreasing mass flow rate and increases with decreasing inlet temperature. In addition, it is found that the channel length has an important effect on the two phase flow dynamic instabilities.