Producing nanofibers of a desired predetermined diameter is important for all applications where the nano-sized dimension plays a key role. In this research, nanofibers from a blend of lignin and recycled poly(ethylene terephthalate) (PET) were prepared using the electrospinning process. The design of experiments (DoE) statistical methodology was employed to screen the significant factors and optimize the whole process. A fractional factorial design with five electrospinning variables (spinning distance, solution concentration, voltage, lignin ratio and flow rate) was implemented to identify their effect on the average fiber diameter and on its standard deviation. The morphology of the electrospun mats was examined using Scanning Electron Microscopy. The statistical analysis of the measurements revealed that only the spinning distance and the concentration of the spinning solution have significant effect on the two responses. To minimize the average fiber diameter, the method of steepest descent was applied in two distinct experimental areas. After lowering the solution concentration and the spinning distance up to the point of bead formation, the average fiber diameter was minimized to 191 +/- 60 nm. Following the method reported here, tailor-made lignin/recycled PET nanofibers of a set, desired diameter can be fabricated by properly adjusting the electrospinning variables.