This study aims to explore the effects of fuel composition on spray ignition using a constant volume vessel. Ignition delays of two-component fuels (n-alkane + HMN) with a wide range of n-alkane volume percentages were measured under two ambient conditions. Based on the results, estimated cetane numbers (CNe) of n-alkane were calculated by the equation, CNe = CNllmN + 100(CN,,佑NHmN)/r. The results showed that the dependency of ignition delays on n-alkane volume percentages changed at a certain percentage for all fuels. The dependency of CNe on n-alkane percentages was considerably affected by ambient pressure.