The mechanical behavior of electrospun PAN nanofibers was measured in tension by a MEMS-based mechanical testing platform. The effect of strain rate on the fiber mechanical response was investigated at three strain rates (10 -2, 10 -3, 10 -4 s -1). For most samples, the engineering stress-strain curves were elastic-nearly perfectly plastic. The engineering strength and ultimate strain were in the range of 40-130 MPa and 60-120%. As the original fiber diameter decreased, the maximum (saturation) stress increased at all strain rates. The ultimate strain was rather insensitive to the initial fiber diameter, but it increased as the loading rate was reduced which may be explained by the enhanced contribution of creep at lower rates. However, the engineering strength did not vary monotonically with the strain rate. This behavior was attributed to a cascade of localized deformations and multiple necking at faster rates and enhanced macromolecular alignment and uniform drawing at the slowest strain rate.