Abstract: As a key physical process of lightning protection on wind turbine blades, the inception mechanism and criterion of the upward leader need further. A model of line charge with point charge at the head was used to simulate charge distribution of lightning leader. A curve about surface charge density on blade was plotted after considering the movement, diffusion and neutralization of the space charges, and positive charges was concentrated distribution at the tip of blade. The potential nearby blade was simulated by COMSOL, and it was distorted obviously under the influence of charges in lightning leader and that on the surface of blade. The paper proposed a critical length inception criterion for upward leader based on a simplified model of the development of upward leader. The kernel of the criterion is ‘there exists a certain zone above blade tip where the background potential is larger than that in streamer, and a stable upward leader will incept if the length of this zone is larger than a critical length’. Lightning parameters and blade parameters, such as amplitude of lightning current, lateral distance, length of blade and radius of receptor, have no effect on the critical length. The critical length decreases with the increase of the relative density and humidity of air, and it increases with the increase of altitude. The feasibility of the proposed criterion is verified by the observation results of long air gap discharge for the vertical conductors under laboratory conditions.