Abstract: In oil-air lubrication lubricants are supplied by micro oil droplets to tribo-pairs. With supplying a single oil droplet an EHL model is proposed to numerically simulate the transient process of building lubricating film. The results show that entrainment speed and oil viscosity have significantlyeffects on the expansion distance of a micro oil droplet, and then led to different film generation behaviors in the contact zone. The higher entrainment speed or the bigger oil viscosity was, the smaller the expansion distance of micro oil droplet was. Consequently oil film emerged only in the middle of the contact zone. The micro oil droplet just as hard particles went through contact area with dimple-shaped deformation on the solid surface. The theoretical results had good agreement with the experimental results.