Abstract: In this paper the Wiberg bond index of olefinic double bond in 8-methoxypsoralen and tetramethylethylene, 1, 1-methylethylene, trimethylethylene and acrylonitrile have been calculated by using CNDO/2 method. The calculated results indicate that the rate of photocyeloaddition varies linearly with the Wiberg bond index of olefinic double bond. It shows that the reaction rate is determined by the Wiberg bond index of olefinic double bond.