Efficient and stable atrazine removal could be realized after a start-up period in a membrane bioreactor (MBR) bioaugmented with genetically engineered microorganism (GEM) and the removal efficiency was above 90%. The start-up period was the key step of MBR bioaugmented with GEM and determined by operation conditions. The shortest start-up period was 2 days and the longest one was 12 days under different operation conditions. Initial influent atrazine loading, operation temperature and initial density of GEM affected the start-up period greatly. High initial influent atrazine loading, high operation temperature and large initial density of GEM were favorable to shorten the start-up period. The characteristics of the influent had little effect on the start-up period. Two kinds of influents - synthetic wastewater and real domestic wastewater, had the same start-up periods when other conditions were identical. The density of GEM varied remarkably in the start-up period: it decreased rapidly at the initial time and then tended to a constant. The logistic model was used to fit the density variation of GEM. The different values of model parameters accounted for the relationship between the atrazine removal performance and the density variation.