A novel broadband directional coupler based on an asymmetric dual-core photonic crystal fiber (PCF) is proposed. The asymmetry in the fiber is introduced by the enlargement of one air-hole in dual-core PCF. Numerical investigation demonstrate that broadband directional coupling with spectral width as large as 370 nm and polarization-dependent loss and uniformity lower than 0.2 and 0.5 dB, respectively, can be achieved. In addition, the proposed fiber shows large tolerance to the variation of the fiber parameters. In particular, the fiber length allows at least 10% derivation from the proposed fiber length of 7.7 mm.