The diameter of the nanofibers produced by electrospinning is a key parameter for their potential applications. In this study, electrospinning was used to produce polyacrylonitrile (PAN) nanofibers at varying parameters of solution concentration, applied voltage, spinning distance, surroundings temperature, and needle diameter. To investigate the effects of these parameters on the fiber morphology and diameter, wide range of them were selected and 24 systematic experiments were carried out. The results revealed that the solution concentration had a significant influence on both morphology and diameter of the nanofibers. With increasing solution concentration from 7 to 19 wt %, the morphology was changed from beaded fibers to uniform fibers and the fiber diameter noticeably increased, ranging from 84 to 757 nm. In addition, solution properties such as viscosity and surface tension at different concentrations were measured for a thorough examination of the solution concentration effect. Also, increasing applied voltage and spinning distance resulted in a minimum in fiber diameter. Moreover, the diameter of nanofibers decreased with an increase and a decrease in surroundings temperature and needle diameter, respectively. Optimum conditions for fabricating nanofibers with minimum diameter and best morphology were determined and PAN nanofibers with diameter of 88 nm were produced.