At the magic angle, the time_of_flight mass_spectrometer (TOF-MS) profile depends only on the translational energy distribution of photofragment, and is indep endent of the angle distribution of recoil momentum, therefore, the translationa l energy distribution is usually determined by fitting the measured TOF-MS profi le. Different from the above procedure, we take another approach in the present work. According to this method, by measuring three widths at half maximum (t 1/2), one-fourth maximum (t1/4) and three-fourth m aximum (t 3/4) of the TOF-MS profile, the averaged translational energy, th e width of translational energy distribution, as well as the width of instrumental func tion can be determined from numeric tables or mathematical expressions. In this method, both the translational energy distribution and the instrumental function are described by Gaussian functions.