May I know how is the weather in Taiwan in December ? Is it hot ? Cold or just cool ? I need to pack

The island covers a large area, so which specific place do you want to go?
In general, the average temperature is around 17 to 18C on the island. It may be a little bit humid during that season.
Thick coat and thin sweaters are necessary and the rain gear is suggested to prepare.

Thanks. I will go to Taipei . Need your advice

The average temperature is around 16 to 19C for Taipei and it may rain a lot in that time.
For the day time, the jeans and T-shirts are OK, but at night, you may need the thin sweaters.