Literature on saturated flow boiling heat transfer in vertical microchannels with modifying the internal surface is limited. The objective of present study is to study saturated flow boiling performance of nanostructured and bare surface vertical micro-channels experimentally. Experiments were carried out in a copper tube having inner diameter of 1.07mm and length of 200 mm, using de-ionized (DI) water. Saturated flow boiling, along with convective heat transfer measurements were performed for the mass flux range of 495 - 891 kg/m2.s, and heat flux range of 71-285 kW/m2. In bare surface microchannel in two high mass fluxes of 671 and 891 kg/m2.s, the dominant heat transfer mechanism was nucleate boiling because the heat transfer coefficient was independent on mass flux and vapor quality but it decreased with increasing heat flux because of microchannel compact geometry that cause confinement bubble. Confinement bubble cause local dry outs in saturated flow boiling. In bare surface microchannel in mass flux of 488 kg/m2.s the heat transfer coefficient was independent on heat flux so the dominant boiling mechanism was forced convective boiling. In nano structured surface the critical heat flux enhanced about 15% because of the increasing wettability but heat transfer coefficient decreased about 20% because of filling the porouse in the channel surface and decreasing the nucleate sites. Keywords: Heat transfer, Micro-channel, Flow boiling, Nano structured surface