Formulation of diesel spray penetration is one of the basic study of diesel spray.Since physical relationships among various injection parameters of diesel spray are included in spray tip equation,it becomes a fundamental "tool-equation" for diesel spray analysis and improvement of diesel spray combustion.There have been many proposals of spray tip equation but most of them were derived from experimental data obtained by classical jerk injection system.However,they have received some critical comments of mismatch with experimental results of up-to-date injection system.Since diesel spray injection system has been changed from jerk pump system to high pressure common rail system,empirical equation of spray tip penetration have to be modified or optimized for new injection system.Here,spray tip penetration equation obtained by Hiroyasu and Arai is modified to match for common rail injection system with high pressure rise.It shows that initial injection rate had strong influence on spray tip penetration.Spray angle and air entrainment have direct relationship with diesel combustion.However,there is no clear analysis on these relationships.Then the additional derivation of spray angle and air entrainment was carried out based on the new equation of spray tip penetration.