Study on composition of NaBH4 solution was performed to increase energy density of fuel cell system for small unmanned aerial vehicle(UAV). First, self hydrolysis test and hydrogen generation test with various composition of NaBH4 solution were performed. Composition of NaBH4 solution(1 wt% NaOH + 25 wt% NaBH4 + 74 wt% H2O) was determined for long time operation based on loss of hydrogen gas, stable supply of NaBH4 solution, NaBO2 precipitate, operation condition of small UAV. Hydrogen generator produced hydrogen gas for 200 W fuel cell powered UAV during 3 hour. Conversion efficiency of hydrogen generator was improved to 87.4% by decreasing the amount of NaOH and increasing the amount of water. However, hydrogen generation rate slightly decreased owing to increase in the amount of NaBO2 precipitate during long time operation.