In order to investigate the influence factors of zero excess activated sludge (EAS) process by ozona tion, a 100 L membrane bioreactor coupled with a sludge ozonation unit (MBR-SO) was performed for 80 d without EAS wasting. Some mathematical models were developed to elucidate the relationship between process parameters and the operating effects. It is considered that the sludge lysing ratio (ζ) , produced COD per unit mass lysed MISS (η), observed sludge yield coefficient (Y_(obs)) and intrinsic yield coefficient for COD produced by lysed sludge (Y_2) significantly affect the flowrate to ozonation unit (q). When q is 0. 0067 times of influent flowrate (Q) and ζ is about 0. 72 for each batch ozonation, a relatively stable MISS concentration of 8168 mg/L and zero Y_(obs) are achieved in the MBR-SO system. The calculation of sludge disintegration number indicates that the high ζ can apparently decrease the sludge amount needed for ozonation. At the same ozone dose, the low input ozone concentration and high flowrate can enhance the sludge lysing effects and a low energy consumption of 0. 041 Yuan/m~3 wastewater is obtained.