Polyacrylonitrile (PAN) nanofibers are very important to achieve high performance carbon nanofibers. In this work, co-polyacrylonitriles (co-PANs) with different molecular weights were synthesized by a simple free-radical polymerization. The effect of the initiator amount on the molecular weight of co-PAN was investigated. The co-PANs with different molecular weight were electrospun into aligned nanofibers by adjusting the absolute viscosity of co-PAN solution into similar to 1.0 Pa.s. All the co-PAN nanofibers showed smooth surfaces and homogeneous fiber diameters of similar to 450 nm. Tensile tests were applied to evaluate the mechanical properties of electrospun aligned co-PAN nanofibers. The results indicated that higher molecular weight led to better mechanical performance of electrospun aligned co-PAN nanofibers. When the molecular weight was 2.3 x 10(5), the highest strength of 153 MPa, strain of 0.148, and toughness of 16.0 J/g were obtained. These electrospun aligned co-PAN nanofibers could be good candidates for the preparation of high performance carbon nanofibers.