This paper analyses several well-known class-cohesion measures in depth and points out their defects. Above all, it states that a class exists in the context of a software system and cannot be isolated because of the existence of interactions between it and the environment. It should be considered that external factors and other classes are capable of affecting the cohesion-measure result for a class. Therefore, we propose a new weighted-LCOM approach to measure class cohesion. Several commonly used measures and the new weighted measure are applied in some experiments to obtain cohesion results, and we prove the superiority, accuracy, and validity of the new measure by comparing these experimental results. The new method represents a new concept for evaluating class cohesion.