I'm going o Taiwan(round island) from 23to 29 Dec 15. What sort of clothing shd I bring? Victor

Oh the round island tour? Well, the temp in northern and southern part are very different. In general, you can wear long-sleeve t-shirt in the southern part; in northern part, an additional overcoat will be needed. If you are going to the mountain area, even warmer clothes will be required like a sweater.
BTW, it tis the rainy season and bring with you the rain gears.