Taipei is facing serious traffic congestion problem. In order to increase the bus ridership, the two contra-flow bus lanes had been implemented on two major arterial streets since February 1990. Department of Transportation, Taipei City Government had imposed another seven exclusive bus lanes from January to August in 1996. This study intend to assess the effect of the bus priority strategy on buses' level of service and ridership. We collected (1) secondary hand data, (2)questionnaire survey data and (3)field investigation data. This study used the statistical test procedure to draw an appropriate conclusion from sample data for the difference between two population parameters (before and after). The ARIMA (Auto-regressive integrate moving average) method, a time series model, also be developed for evaluating whether the bus ridership is increased or not. The results showed that (1) "efficiency" and "convenience"--two factors of buses' level of service--got positive effect, "safety" got negative effect in initial stage and got positive effect after one year, (2) the ridership of buses showed significantly increased after the bus lanes implemented.