1.An X-band E.S.R.spectrometer has been constructed.Its sensitivity is such that 3×10^-8 mole of DPPH can be detected by oscilloscope with S/N=15/1.The five hyperfine structure lines of 5×10^-4 M/1 DPPH benzene solution have been resolved.2.Both transmission and reflection detecting system are provided.The former is used for the detection of high frequency signal and automatic tuning signal,and the latter for crystal vedio detection and microwave monitoring.This makes it more convenient to set the spectrometer for use.3.A single loop modulation coil used inside the H₀₁₁ type cylindrical cavity is described.Satisfactory modulation can be obtained with a low power high frequency oscillator so that serious interferences due to modulation pick up has been eliminated.