Several familiar methods for evaluating beam quality are discussed, and the shortcoming of them used in evaluating beam quality of laser diode is also analysed. The main factors to influence collimating the beam of laser diode with lenses are analysed. A new quality parameter(QP) lθ5(l is the size of the emitter in the direction parallel to the junction plane, 9 is the angle of the beam in the perpendicular direction.) for evaluating beam quality of laser diode is given. The difficulty of collimation is well expressed by this mean. The smaller the value of lθ5 is, the more easily the beam is collimated. Furthermore we give a reference value of the QP, when the QP value of the beam is smaller than this value, this beam can be collimated without much difficulty.