Ultrasonic velocity and absorption in liquid diiodomethane have been measured in the frequency range 100 MHz~1.7 GHz at the temperatures of 10° and 20°C using the highresolution Bragg reflection method. Vibrational relaxation was observed which was ascribed to all the vibrational modes except the lowest. Vibrational relaxation frequency β in fifteen liquids obtained in our previous and present works is plotted as a function of fundamental vibration frequency ν of the relevant trigger mode. The relation log β=A-kν well fits the data within the factor of two, where A and k are constants. This indicates fundamental vibration frequency is a dominant factor in determining relaxation frequency.