We developed a PCI interface for GRAPE systems. GRAPE (GRAvity piPE) is a special-purpose computer for gravitational N-body simulations. A GRAPE system consists of GRAPE processor boards and a host computer. GRAPE processors perform the calculation of gravitational forces between particles. The host computer performs the remaining calculations. The newest of the GRAPE machines, GRAPE-4, has achieved a peak performance of 1.08 Tflop. The GRAPE-4 system uses TURBOChannel for the interface to the host, which limits the selection of the host computer. The TURBOChannel bus is not supported by any of recent workstations. We have developed a new host interface board which adopts the PCI bus instead of the TURBOChannel. PCI is an I/O bus standard developed by Intel. It has a fairly high peak transfer speed, and is available on a wide range of computers, from PCs to supercomputers. Thus, the new interface allows us to connect GRAPE-4 to a wide variety of host computers. In test runs with a Barnes-Hut treecode, we found that the performance of the new system with the PCI interface is 40% better than that of the original system.