Electrochemical supercapacitors based on metal oxide nanostructures are poised to provide integration of power sources onto electronic devices, but they suffer from poor cycle lifetime, as well as low energy and power density. To tackle these problems, we report a simple, low-cost, safe and broadly applicable hierarchical bottom-up assembly route for the formation of ultrathin (3nm) vanadium oxide (V2O5) film on conducting MWCNTs. The ultrathin V2O5 shows a very high capacitance of 510Fg−1 and possesses excellent cyclic stability with negligible decrease in specific capacitance after 5000 cycles. The supercapacitor device based on this hierarchical bottom-up assembly exhibit an excellent charge/discharge capability, and energy densities of 16Whkg−1 at a power density of 800Wkg−1. Such high performances are due to both the high crystallinity and complete accessibility of ultra-thin film of V2O5 to the aqueous electrolyte of 2M KCl.