The electron mobility of 4,7-diphyenyl-1,10-phenanthroline (BPhen) at various thicknesses (50–300 nm) has been estimated by using space-charge-limited current measurements. The measured bulk mobility is in excellent agreement with results from time-of-flight method. It has been observed that the electron mobility of BPhen approaches its true value when the thickness is more than 150 nm. The estimated electron mobility of BPhen at 300 nm is found to be 3.4×10-4 cm2/V s (at 0.3 MV/cm) with weak dependence on electric field. For thickness typical of organic light-emitting devices, the electron mobility of BPhen is also investigated.