A co-axial microfluidic device was designed and assembled, and the micro-droplets of 100~600 mm with good spheric shape were successfully prepared. The test shows that the droplet diameter dp was mainly determined by the tapered tip Dt (40~450 mm) of the microfluidic devices. For a given tapered tip, the dp decreased with increasing flow rate Qc of the continuous phase. The increase in the flow rate of the dispersed phase only had a minor effect on the droplet diameter, but increased the frequency of micro-droplet generation. Based on the test, the correlations between dp and Qc for the device of different sizes are presented and their correlation coefficients are all above 0.99. Then the rising velocities of micro-droplets of soybean oil and toluene in stagnant liquid were measured. The results show that, unlike the millimeter-scale droplets which usually have lower drag coefficient CD than the rigid particles due to deformation, the relation of CD versus Re for micro-droplets is consistent with that for the rigid particles and the reason for this finding is given in terms of interfacial tension. Observation shows that, due to the wake effect, the bulk velocity of the droplet swarms is 30% higher than the single droplet with the same diameter.