This study focuses on the turbine performance and internal flow field of a newly developed cross-flow turbine which has a cavity and a guide wall. The cavity connects at the edge downstream an outside nozzle wall, and the guide wall connects the cavity. In typical cross-flow turbine, the circumferential velocity component is active in the outflow from a runner. The developed turbine can change the velocity component to radial direction by the cavity and the guide wall for performance improvement by enhancement of differential angular momentum and size reduction. The authors revealed that the relationship between internal flow fields and the runner performance by lab test and CFD analysis. As a result, performance tests revealed the maximum and the partial load turbine efficiency under the conditions of various guide vane opening and runner rotation speed. The efficiency becomes the maximum at guide vane opening ratio of 86%. The minimum partial load efficiency is approximately 45% of the best efficiency for the guide vane opening ratio of 14% when the runner rotation speed is constant. Flow visualization results by the experiment and CFD analysis showed a significant role in changing the outflow direction by the guide wall and cavity.