[1] Fast reconnection is crucial to magnetospheric substorms, solar and stellar flares and fusion plasmas. Ultimate confirmation of fast reconnection must be achieved by multi-spacecraft detections of the reconnection rate itself and associated dimensions of the diffusion region. Here we report a multi-spacecraft measurement of fast reconnection rate grec Vin y (0.07-0.15)VA based on directly measurements of the plasma flow into the diffusion region, where Vin is the speed of reconnecting flux and VA the characteristic Alfven speed. It falls in the range of (0.03-0.2)VA predicted in steady state reconnection simulation. The characteristic sizes for the diffusion region of the width Lz 0.9 di(=460km) and the length Lx (3.3-5.1)di (=1680-2597 km) are measured as well. The length of the diffusion region Lx is determined for the first time based on the in situ observations. Furthermore, other features detected during the event also match the previous observation and simulation results. Citation: Xiao, C. J., et al. (2007), A Cluster measurement of fast magnetic reconnection in the magnetotail, Geophys.