We use electron spin resonance ( ESR) to study the properties of the unpaired electron in an organic chemical compound 2，2 -diphenyl-1-picrylhydrazyl ( DPPH) ，including its hyperfine coupling with the nucleus and its Zeeman splitting under magnetic field. The Lande-g factor is obtained by measuring the resonance magnetic field and microwave frequency. A magic-T waveguide is applied to increase the signal-to-noise ratio. The Lande-g factor is measured to be 2.033±0.0003，which agrees well with the expectation reference value 2.0036 found in other experiments. This method can be used to study the unpaired electrons in various materials and biosystems. And it also provides a promising method to study the radicals of micro-inductors on chips.