Transplantation of a single oil droplet was observed in a pure rolling ballondisc contact via an optical interferometry system.The dynamic lubricating processes of film formation and different film regions during transportation were analyzed.A spreading ratio was defined to quantitatively describe the influence of droplet size and speed on lubricating performance.The results show that the spreading ratios excesses 70% for large droplet with slow transportation,indicating spreading effect is beneficial for lubrication duration.However,the spreading ratios will be smaller than 30% for the quick transportation of a small droplet,which indicates the limitation of spreading effect.An inlet oil reservoir is found to be formed by a large droplet through experimental observation,which contributes to a classical film shape,whilst a film shape with dimple is obtained by a small droplet with quick passage.