be careful of , 和be careful about用法大体一样：注意……（多指注意要做到某事,比如注意自己或他人的健康等）

be careful with 是指,对……小心.（多指小心不要对某物或他人造成损害或者伤害）

Please be careful with the vase because it is very fragile.

You are very careful about your diat.