The relationships between the octane number and the carbon atom number and the molecular structure of alkanes were comprehensively analyzed by using the detailed kinetic model generated by there automatic reaction scheme generation tool, KUCRS [1, 2].The octane number is an index showing the ignition delay in the engine temperature regime, that is, the engine ignition temperature range. The high octane number is observed in the following two cases; 1The ignition delay of the low temperature region is large.2The ignition delay of the low temperature region is the same, but the transition temperature for NTC (Negative Temperature Coefficient) region is low.Classifying the reaction path into the main path leading to the chain branching, and the four branching chain reaction prohibition paths, the reaction processes of various alkanes were analyzed;(M1-M6) Main chain branching path: RH+OH→R·ROO·→Q·OOH→·OOQOOH→HOOPO→OPO+2OH(P1) Prohibition path 1 : R•→Alkene + R′•(P2-1) Prohibition path 2-1: ROO•→Alkene + HO2(P2-2) Prohibition path 2-2: Q•OOH→CyQO(Cyclic ether)+OH(P2-3) Prohibition path 2-3: Q•OOH→Alkene + HO2Changes in the octane number by the molecular structure can be explained by the transition temperature to the NTC range. This process is controlled by (P1), the Prohibition path 1.The increase of the carbon number of normal alkanes enhances the contribution of (P2-2), the Prohibition path 2-2 which generates OH at the same time. The Prohibition path 2-2 shows peculiar characteristics. It attenuates (M1), the initiation reaction of the main branching chain path and it generates the branching chain carrier, OH. (P2-2), the Prohibition path 2-2 plays the predominant role in generating OH during the LTO (Low Temperature Oxidation) preparation period. When the carbon atom number is large, (P2-2), the Prohibition path 2-2 is enhanced, and OH generating rate increases, resulting in the lower octane number.