This paper described a method for the identification of protein through its composition. The basic idea is as follows: A database for lengths, compositions and molecular weights of all known proteins is easily derived from a protein sequence database and by comparing the composition, length and molecular weight of the protein in question with each array of data in the composition database, proteins with similar length, composition and molecular weight may be found; as such a tentative identification for the said protein may be made. In some cases, it can be perfectly accurate to predict the right protein(s) from the composition database. A computer program applying this method was developed and used to search for the compositions of human insulin precursor and other proteins. The results are good, which proves that it is a fast and economic way to identify a protein based on its amino acid composition.