Objective: The purpose of this study was to use time series analysis methods to examine the impact on cases of ambulatory care, claims of ambulatory care and the grand total of medical claims when co-payment and regressive physician payment policies were implemented. Methods: Based on the data from the NHI from Mar. 1995 to Dec. 2001, time series models were used to formulate the time series. Results: By selecting the appropriate time series models, the R squares of these models were over 90 percent except for the model of ambulatory care cases. The two interventions including co-payment and regressive physician payment policy were used in the intervention models. The result showed that co-payment intervention had a delayed effect of one month. Conclusions: The results were similar with that of existing papers in terms of descriptive analyses. The outcomes of our models were also reliable in the point estimation in the forecast of 2002 expenditure. The outcome of intervention models found that regressive physician payment policy had a greater impact than the co-payment policy in the month when the policy was implemented, but the predication of one year using the time series models produced different results.