Microfluidic fuel cell (MFC) is a considered as a promising energy source for powering portable electronic devices due to low cost and easy control. However, the energy density of MFC is much lower than conventional batteries like lithium battery. Due to the fuel crossover, the optimized operation concentration remains 1-2 M. In this paper, a counter-flow MFC is presented to achieve concentrated fuel. Density mismatch between fuel and electrolyte is the key cause for fuel crossover. Experimental and modeling study confirms that through adjusting different channel design according to fuel concentration, fuel crossover can be prevented.