A method of controlling a low voltage DC-DC converter (LDC) of an environmentally-friendly vehicle includes measuring load current consumed by electric loads of the vehicle; identifying a state of an auxiliary battery vehicle; and when the load current is smaller than a predetermined value, and the state of the auxiliary battery is equal to or greater than a predetermined value, performing a first eco mode of stopping operation of the LDC for a predetermined time, and causing the auxiliary battery to supply the current necessary for the electric loads.