is Taiwan good to visit on September? any monsoon season ?

Well, to be honest, June to October every year is the typhoon season. There may have several heavy rain days that may influence the sightseeing a lot. Thus if possible, you are advised to change the travel date.

Hey!
I think you should come to Taipei after September around October. At that time, the weather conditions improves and you can get all the fun during your trip. If you come before that,there may b a possibility of typhoon and you may have to stay indoors during that time. So its better if you plan your trip a little later.