Hydrocarbon thermal cracking reactions are the key steps in petroleum refinery processes, and the mechanism is generally accepted to involve free-radical reactions. The MP2 ab initio method was applied to study β-scission reactions of propyl, butyl, and neopentyl radicals. The CBS-RAD(MP2) method was created as a modification of the CBS-RAD method. The method had the least root mean square error for heats of reaction and activation energy calculations for 15 hydrocarbon cracking reactions with less computational cost compared with the G2 and G3 methods. Work applying Transition State Theory and RRKM theory was done to estimate the reaction rate constants for hydrocarbon cracking reactions. The CBS-RAD(MP2) method had excellent agreement with the experimental data, indicating its reliability as a method for studying other large hydrocarbon cracking reactions. A reaction kinetic model with pressure and temperature effects was also proposed, which could be easily applied to different reaction conditions without performing additional expensive and complicated calculations. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).