Letters of guarantee between extramarital lovers generally have no legal effect and are not protected by law. But it also depends on the actual content of the letter of guarantee. As long as the intention of the parties is true, it does not harm the interests of the state, the collective, the third party and the public, and the content, form and conclusion procedure of the letter of guarantee are not illegal (not violating the mandatory provisions of laws and administrative regulations), it should have corresponding legal effect.