The purpose of this study is to propose the three methods for estimating the reliability coefficient under Item Response model. The first method may be used for the item selection, if the latent trait distribution is assumed to be known. The second method gives the upper bound of the reliability coefficient estimate. By the third method, the estimate of reliability coefficient may be calculated from the two following sample statistics only: the sample mean and variance of the estimated values on the latent trait. Some examples using the Scale for Word Meaning Comprehension (Shiba & Noguchi, 1982) are given as application.