We investigate quantum transport in the presence of an electron reflection grating fabricated in an electron wave transistor structure. The grating is made up of a periodically corrugated potential wall by which the electron waves are coherently scattered. We observe a number of peaks with respect to the gate voltage in the low-temperature conductance measurements. The conductance oscillations are attributed to the electron diffraction effect, and the peak positions agree well with those predicted by the Fraunhofer diffraction condition. (C) 1999 Academic Press.