This present work investigates the effects of various tooth profile modification on tooth engagements of a pair of helical gears on the plane of action, tooth lead crowing and transmitted torque on tooth mesh stiffness (TMS), by using existing finite element software on computer. Furthermore the effects of profile modification and lead relieving on contact stress (CS) and mesh stiffness (MS) is investigated. Tooth surface contact stress (TSCS) of a pair of helical gears with mating error (ME), assembly error (AE) and tooth modification (TM) are analysed by the FEM software. Effects of ME and TM on TSCS are also defined as “Influence factors” and the influence factors of ME and TM on TSCS are calculated quantitatively. © Research India Publications.