[en] The PEFP (Proton Engineering Frontier Project) RFQ Accelerator was constructed to accelerate 20-mA CW Proton beam from 50 ke V to 450 ke V. This 350- MHz RFQ is to form with one 98-cm cavity. It is essential that the field distribution meets the design requirement in order to minimize the beam losses. The results of experimental studies on RF characteristics of a four vane RFQ cavity was obtained. A mode separation and field distribution have been obtained with a loop coupler. The tuning procedure was performed by employing the bead-pulling technique. In this technique a metal bead was suspended on a nylon line and drawn through the four quadrants near the RFQ's outer wall. Sixteen Aluminum slug tuners distributed along the outer walls tuned the RFQ. By tuning the positions of 16 slug tuners, a uniform field distribution within errors a few percent both longitudinally and azimuthally were achieved in this RFQ