The most common gear wear mechanisms, such as micropitting, pitting and scouring often occur in the early stage of failure. For the geared system operating in the presence of such surface failures, the load acts only over a finite region and contact stress tends to increase enormously in the contact area of the mating teeth surface. The propagation of tooth damage causes instantaneous reduction in tooth stiffness. The vibration signal of gear transmission varies as the stiffness changes. Therefore the connection between surface fatigue failure, stiffness and vibration signal are of significant importance in monitoring gear defects. The purpose of this study is to conduct experimental investigations on the measurement of reduction in the gear teeth stiffness along with vibrations parameters. A pair of spur gears was tested under an accelerated test condition in an oil bath lubricated back-to-back gearbox. Experimental measurement of stiffness was carried out using modal analysis in conjunction with a theoretical model. It is concluded that stiffness measurement exhibits a direct relationship with the propagation of surface wear and the increase in vibration amplitude of the gear transmission system. Hence this procedure can be concluded as a suitable technique in health monitoring of gears. © The Author(s) 2011 Reprints and permissions: sagepub.co.uk/ journalsPermissions.nav.