Abstract: This paper presents experimental results on an L-band relativistic klystron amplifier in Sinus 700 accelerator. It includes the modulation of intense relativistic electron beam(IREB) after driving through gaps of the input-cavity and the idler cavity, and microwave extraction from the modulated IREB. Annular electron beams of voltage 750kV, current 8.6kA, diameter 57mm, thickness 2mm were produced from annular foilless diode, and 2.1GW/15ns,1.7GW/18ns,0.75GW/30ns output peak microwave power were observed after three cavities,respectively. The efficiency is 14%～33%, and the gain is about 32～40dB.At the same time, microwave pulseshortening was found in experiment．