Abstract: In educational and psychological tests, reliability is an important index judging whether a test is reliable. In this paper, reliability estimators of tests and its statistical properties were considered under different test models.In normal model, an unbiased estimator of reliability was obtained; in binomial model, a consistency estimator of reliability was given; based on the Item Response Theory, another category estimator of reliability was found;in composite test model, a best lower bound of reliability so far was provd, at the same time, its estimator was given.