Many small hydropower stations, which are run-off river types, use a cross-flow turbine because of its excellent partial load characteristics and low manufacturing cost. So far, there have been various studies to improve turbine performance. On the other hand, the flow field pattern is expected to be different with the variable effective head condition. Therefore, understanding the characteristics of flow field patterns and obtaining design guidelines for cross-flow turbines will improve the turbine performance and enlarge operating conditions. This study investigates the influence of the head on cross-flow turbine performance. This study used CFD analysis to evaluate the flow field pattern in the turbine with several head conditions. As a result, it is confirmed that the turbine shows a low degree of reaction. The first stage generates much torque by velocity head variation. The energy recovery rate at the second stage is changed with the head, which is a major reason for the change in turbine efficiency. The flow pattern around blades at the first stage varies with the head.