How is the weather in Kaoshiung in mid Dec. till end Dec. ?

It will get cool even a little cold at that time, with the general temp range of Kaoshiung of 15-22C, and that for Taipei being 9-18C. Cloudy days would be the majority in the two places based on the past records, and you may still meet some rainy days, which won't last for a whole day. Generally, you can prepare long-sleeved shirt, light sweater, coat/leather jacket, and jeans. Long John and simple rain gear will also help.

Hi ! Thank you and appreciate for your prompt reply.

Hi! we plan to travel earlier next week till early Dec instead of mid Dec.Kindly advise if the temp still range around 15-22C, and that for Taipei being 9-18C? or rainy day?

Well then, it can be a little warmer in both places. The temp for Kaohsiung can be 16-26C or around, and it can be 12-20C for Taipei. Frequently light rain may happen then, so an umbrella should be prepared.