A model describing nanofiber formation in electrospinning of a polymer solution is proposed. It is shown that the final nanofiber diameter is determined by the dynamics governed by two main competing factors: (i) elongation under the influence of the electric repulsion between the charges on the fiber surface, which is opposed by (ii) the viscoelastic forces growing in the course of time due to evaporation and stopping the fiber elongation and thinning. Both scaling and numerical analyses performed show that the terminal fiber diameter is controlled by the solution viscosity, the specific charge (electric current divided by the spinning throughput) and the evaporation rate. Model predictions are supported well by our own experiments performed on polyamide-6 solutions as well as literature data. (C) 2016 Elsevier Ltd. All rights reserved.