Amorphous vanadium oxide (V 2 O 5 ) is a very good candidate as material for cathode thin film since it has a relatively high capacity. In addition, the room temperature deposition process is valuable in V 2 O 5 thin film fabrication. Due to these advantages, much effort to grow amorphous V 2 O 5 thin film has been made. In this research, we successfully grew amorphous V 2 O 5 thin film using room temperature sputtering. Based on a Li/LiPON/V 2 O 5 full cell structure, charge-discharge performances were measured according to cycling number. Even though the full cell structure showed an average capacity of 15 μAh/cm 2 over more than 500 cycles, a capacity fade was shown after a few cycles. Many reports revealed that the phase change of V 2 O 5 from amorphous to crystalline made this kind of capacity fade. In order to investigate this phenomenon, high-resolution transmission electron microscopy (HRTEM) was employed. The as-deposited V 2 O 5 thin film consisted of a homogeneous amorphous without any grain-boundary and/or polycrystalline island. However, the microcrystalline V 2 O 5 phase was randomly distributed in the amorphous V 2 O 5 thin film matrix after 450 cycles by cross sectional TEM (XTEM). That is, some amorphous phase in the V 2 O 5 matrix changed to the crystalline phase. This crystalline phase strongly prevented the extraction of Li ions during the charge process, which induced the irreversible diffusion of Li ions from cathode to anode. From this result, a high efficiency thin film battery based on amorphous V 2 O 5 can be fabricated by preventing amorphous-crystal phase transformation during cycling.