A Rutherford flat cable that is applicable for operational currents up to 1000 A at 13-14 T was developed using a bronze-processed high-strength Nb 3Sn strand with CuNb reinforcement (CuNb/Nb3Sn). The critical current for a 0.8-mm diameter CuNb/Nb3Sn strand is 98 A at 14 T and 4.2 K in the residual strain state. A test coil using the CuNb/Nb 3Sn Rutherford flat cable composed of 16 CuNb-reinforced Nb 3Sn strands was fabricated. We measured the critical current properties of the Rutherford test coil and obtained an excellent critical current of 1840 A at 14 T and 4.2 K. By using the strain gauges attached onto the stainless-steel reinforcement tape that was co-wounded with the Rutherford flat cable, it was found that a 300-MPa hoop stress at 4.2 K was applied to the CuNb/Nb3Sn strand. This implies that the critical current for a CuNb/Nb3Sn strand is enhanced to be 115 A at 14 T and 4.2 K through the stress-strain effect of the critical current at 300 MPa.