In this study, the formation of droplets from a micro capillary is simulated by a Front-Tracking / Finite Difference method under the circumstance where the ambient fluid co-flows with the dispersed liquid. The shape of the interfaces as well as the size of the resulting droplets, are well compared with experiments by Utada et al. and with computations by Hua et al. [AIChE. J., 53, 2534 (2007)]. Extensive numerical experiments are carried out in order to find the transition from dripping to jetting. The transition points obtained by our computations are also in good agreement with experiments. In jetting mode, it is confirmed by numerical simulations that the uniform velocity distribution is developed inside and outside the jet due to the dragging by the co-flowing ambient fluid. This situation corresponds to the assumption of linear stability theory, and thus supports the fact that the size of the resulting droplets in jetting mode is in good agreement with the one predicted by linear stability theory.