回复：突然想起前几天的一个帖子，如朋友偷东西自己不揭发，算是犯罪吗？

Depends on where you are and how much your friend stole. Under US statute, if you witnessed a felony and did not report promptly, you committed misprison which could result in fine and up to 3 years of imprisonment. Some states do not have this as this is mainly a common law doctrine. Other states like CA only requires you to report violent felonies.