The process of droplet fusion is vital for numerous industrial applications, including printing and sintering, as well as chemical and biological processes like emulsification and extraction. Recent advancements in droplet microfluidics research have highlighted the need to further study this phenomenon. This chapter focuses on droplet fusion by creating a cavitation bubble on an optofluidic chip. The discussion covers the working principle, droplet fusion under different conditions, and fusion of droplets with bubble or cell.