Bunch beams accelerated by an S-band electron linac have a very short (picosecond range) pulse duration. However, the beam accelerated by an electron linac has a 350 ps interval fine structure, therefore just one bunch must be extracted from the 350 ps interval bunch train by means of a special injector which is composed of a very fast electron gun, sub-harmonic buncher and pre-buncher, or rf chopper. This report describes the single-bunch operation of an S-band electron linac and its detection systems. At present, the measured bunch duration is 16 ps which is the limit of the detection system, and the measured maximum charge is 1 nC.