Bubble formation from a downward-pointing capillary nozzle was investigated in this study. The experiments were conducted at gas flow rate of 40-5400 mL/h and inner nozzle radius of 0.030-0.255 mm. Experimental results show that microbubbles were formed continuously at moderate Weber number, which was not reported in pervious investigations with injecting gas through an upward-pointing capillary nozzle. High-speed visualization indicates that the formation of microbubbles arises from the convergence of the capillary waves induced by the partial coalescence of larger bubbles. A bubbling regime map is given to identify the critical conditions for the formation of microbubbles. In the present air-water experiments, the generated microbubbles are 20-170 μm in diameter. From experimental data, a scaling law for microbubble size is proposed as a function of Weber and Bond numbers.