The purpose of this thesis is to improve the performance of dimmable ballast. The performance includes the following indexes: system efficiency, electromagnetic interference, starting voltage, fluorescent lamp life and power factor. According to the electrical characteristics of fluorescent lamp, a high efficiency of dimmable ballast can be accomplished by a half bridge with series resonant inverter used on off pulse number control of pulse width modulation is made of by microprocessor and logic control circuit. When the operating of ballast is from 80w to 40w, the efficiency is above 90%, the power factor is above 0.99, the crest factor of lamp is under 1.4, starting peak voltage is 350v. These experimental results show that the purposed electronic ballast is able to meet the request of energy saving, environment protection and power quality.