Abstract: Free radicals generation and oxidative stress are involved in many pathological processes including neuro-degenerated diseases, diabetes, tumorigenesis and radiation damage. Measurement of free radicals is of importance in the field of radiation protection and radiation damage. The electron spinning resonance (ESR) techniques for measurement of free radicals in biological systems are summarized according to the literature and the author's research work in this review. The recent emerging imrauno-spin trapping method for analysis of protein radicals and D N A radicals is also introduced.