one of which (= 其中之一) 可以引导定语从句,而 one of them (= 其中一个) 不行,如：
I have five books,one of which is English.(=我有五本书,其中有一本是英语书.) ；
I have five books ,and one of them is English.(意思同上)；
one of which 只能指“物”；而 one of them 既能指“物”,也能指“人”,如：
There are five children in the room,and one of them is reading.(= 房间有五个孩子,其中一个在看书).