Abstract Evacuation leaders (individuals guiding the crowd) can make crowd evacuation more efficient. Selecting their number and positions within the crowd is particularly important when the other evacuees are not familiar with the internal layout of the building. This paper investigates the effects of leadership on crowd evacuation in rooms with limited visibility range by an extended social force model. We find that, (i) For a large evacuees crowd, a small proportion of the evacuation leaders is already sufficient to guide the whole crowd efficiently, even if the visibility range of the room is very limited. And the smaller the crowd size, the larger the proportion of the leaders is needed to achieve a satisfactory evacuation guidance. (ii) The optimal proportion or number (i.e., the proportion or number that is necessary for guiding the crowd) of the leaders in an evacuees crowd to achieve a satisfactory guidance is related to the visibility range of the room and the distribution range of the evacuees crowd. (iii) Leadership is not always positive to the crowd evacuation. It may have a negative effect on crowd evacuation when the visibility range of the room and the size of the evacuees crowd are large enough. The findings provide a new insight into the effects of leadership on crowd evacuation.