Abstract: This paper investigated the influence of rudder gap size on rudder efficiency of different rudder on flying wing aircraft using numerical simulation method. Result indicates that the rudder efficiency of inboard and outboard elevon is reduced because of rudder gap, and the bigger the gap size the more rudder efficiency decreases. Compared with that in no gap condition, the efficiency of split rudder is higher when rudder gap exists. The inboard and outboard elevon efficiency decrease caused by rudder gap can be attributed to the fact that the airflow of lower surface go through the gap to the upper surface, leading to the reduction of the pressure differences between the upper and lower surface, so as to block the main flow. The mechanism of split rudder efficiency increase at high deflection angle can be explained by the fact that high pressure airflow at the lower surface go through the gap and inject into reflux zone at upper surface. This flow phenomenon results in the reduced reflux range.