Instability characteristics generated with partial flow rate in a mixed-flow pump have bad influence during the pump operation. This paper is described on the improvement of instability characteristics in a mixed-flow pump based on flow near blade tip. At first it was examined about the influence of the suction flow channel diameter which gave to performance and internal flow of mixed-flow pumps experimentally. Furthermore, it was examined about influence when it combined control of blade loading distributions near blade tip. As a result, pump efficiency fell at flow rate of the best efficiency a little when we reduced suction flow channel diameter. However, instability characteristics were improved so that a reverse flow developed at the impeller inlet tip side with the decrease of the flow rate slowly. In addition, about 82% head falls of instability characteristics were improved when we combined reduction of suction diameter and the impeller of aft-loading.