After systematic experimental investigations, the chief factors which influence the sensitivity, stability and resolution of the ESR spectrometer have been ascertained. Effective means of improving these characters have been worked out. As a result, the sensitivity has reached 2.7×10-12M DPPH. (RC = 2sec., microwave power into cavity ≈l0mw.). The resolution has been found to be better than 1×10-4. The baseline stability has been greatly increased. The main drawbacks inherent in this type of spectrometer has also been discussed.