This work describes miniature formic acid fuel cell batteries, which are built based on a Nafion® membrane and thin metal foils. The intrinsic advantages of formic acid fuel allow for a very simple design of the fuel cell, and the volume of the complete system, including fuel reservoir, can be as small as 11mm3. This work examines the effect of membrane thicknesses and fuel concentrations on the cell performance. The optimized cell performance is obtained with N117 membrane and 12M fuel. Peak power density of the optimized cell is 112mWcm−2. Life tests are conducted at various conditions using 6μL of fuel. An energy density of 70WhL−1 with 40% fuel utilization rate is observed when 12M formic acid is used at 0.5V.