The purpose of this study was to examine how spectator's effectiveness of ambush marketing influence sponsorship outcomes during the event of 2012 OEC Kaohsiung Men's ATP Challenger Tour. Data was collected via survey questionnaires from all spectators participating in the event during April 25 to April 29, 2012. A total of 304 questionnaires were distributed; 213 questionnaires were considered useful for data analysis after sorting out invalid responses. Data was analyzed by utilizing SPSS 14.0 and AMOS 7.0 statistical software. The results of study indicated that the effectiveness of ambush marketing from non-official sponsor weaken the outcomes of the official sponsorship. Discussions and suggestion for future direction would be also provided.