[en] For TRISTAN, fabrication and tests of five-cell niobium cavities have been carried on. Sixteen five-cell cavities were installed in the TRISTAN tunnel in the summer of 1988. Before the installation of superconducting cavities, TRISTAN has been operating with 104 nine-cell normal-conducting cavities with the maximum beam energy of 28.5 GeV. By adding the superconducting cavities, the beam energy has been upgraded from the Autumn of 1988 to 30, 30.4 and then 30.7 GeV. The second sixteen five-cell cavities were also fabricated, tested and installed in the tunnel at the beginning of August 1989. Cool down and commissioning of the cavities will start at the middle of October, and from November beam energy will be pushed up to 32 or 33 GeV. The present paper reports the test results of thirty-two 5-cell cavities before installation in the tunnel, and some of experience of the beam operation of the first sixteen 5-cell cavities. Vertical test results given include the statistics of accelerating field gradient, field strength for each cavity, RF losses of cavities relative to the peak surface field, etc. Two 5-cells were then combined and assembled in horizontal cryostats with input coupler, HOM couplers and frequency tuner. Some horizontal test results are also presented. (N.K.)