By employing the semiclassical theory of lasers, a relation between the frequency tuning range and various laser parameters is given in this article. It is demonstrated that the width of the tuning range not only depends on the gain profile, but also on the following factors, i. e., the sensitivity of frequency selecting elements, relative level of the laser output power, the linewidth of selected modes, and the operating coniiguration of the laser. The output characteristics of lasers at the critical frequencies, i. e., at the boundary of the tuning range，is analyzed in detail as well. It is shown that the intensity of the selected modes varies slowly when the frequency is tuned in the tuning range, but decreases repidly when it is tuned out the boundary of the -tuning range，that is to say, the energy of the laser oscillator is rapidly transfered from the selected modes to non-selocted modes which will grow up around the centeur of the gain curve. Therefore the common experimental phenomena mentionod above are explained theoretically.