Abstract: A deep analysis is made of the essence and behaviors of lack of cohesion in methods （LCOM） in object oriented software metrics from both theoretic and experimental aspects. Three kinds of LCOM definitions are given. They are α LCOM, β LOCM, and γ LCOM. Their advantages and disadvantages are compared. Theory and experiment show that there is a lack of meanings of degree in α LOCM （i. e. C＆K LOCM） and in β LCOM, γ LCOM has better characteristics but poor stability in a class with a few of methods. Also analyzed are two kinds of LCOM definitions derived from α LCOM, i. e. β′ LCOM and γ′ LOCM. They have the same characteristics as β LCOM and γ LOCM. It is indicated that the research on LCOM should be combined with the research on psychology about programmers.