Business process model simulation is modeled based on Standard Operational Procedure (SOP). Event log is a collection of cases which every element refers to an activity, case, and time of event or process in an information system. This research was conducted in order to simulate business process model utilizing discrete event approach (DES) and process mining paradigm to simulate information from the event log. We obtained event logs from Badan Pengelola Pajak dan Retribusi Daerah (BP2RD or Regional Tax and Retribution Agency) in business tax determination process. Event logs need to be simulate to reflect the performance in real situation. After that we use the existing event log to forecast the performance in indicate years. We utilize exponential smoothing method to forecast the number of business actors for the subsequent years. At the end, we compare existing performance and forecasting performance. This research aims to calculate tax determination performance for business actors according to existing business process, analyze existing performance with DES and analyzing future performance by forecasting business actors' growth. The result of the research shows the differences of performance evaluation between existing and forecasting event log, forecasting log occurs increment 615 logs or 60.12% with execution time decrease to 95.39% each trace.