Abstract: The application of mass spectrometry to investigations of gas-phase photochemistry and kinetics by flash photolysis was first reported in 1957. Mass spectrometry is advantageous and versatile for kinetics and mechanism studies since it is capable of detecting many different chemical species during the course of the reaction and one can determine kinetic rates of reactant decay and product growth. When the mass spectrometer is used to determine kinetic decays and growths following pulsed photolysis, it may be called flash photolysis with time-resolved mass spectrometry(FPTRMS). While experiments using FPTRMS can obtain direct and valuable information on kinetics and mechanism of chemical reactions, they are limited in some cases. For example, experiments can be conducted only at limited temperature and pressure ranges. Moreover, the experimental investigation alone does not always provide sufficient information for the reaction mechanism study, especially for a complex reaction system. The theoretical investigations including Ab Initio calculations, RRKM calculations and numerial simulations are useful and supplemental tools to the experimental studies. Ab initio and RRKM calculations can be used to predict rate coefficients of chemical reactions in a much wider ranges of temperature and pressure. Important information on the mechanism can also be obtained from the theoretical studies. However, the validation and accuracy of the results from the theoretical investigations need to be verified or adjusted by experimental results. As an example, a study of kinetics and mechanism of CF_3CHClO radical reactions using FPTRMS combining with the theoretical calculations is reported.