Abstract: In order to obtain the horizontal restriction of any two aircraft, a method of the assignment problem (AP) of flight level is presented. Based on the expanded efficiency matrix of generalized assignment problem (GAP), Hungarian algorithm provides a solution for the optimization of the vertical trajectory with the efficiency matrix of the flight level from one single aircraft to multiple aircraft in the same flight level. It also provides a general method to convert the indeterminate mode of assignment problem to determinant mode when the efficient matrix varies. The case study shows that the direct or indirect Hungarian algorithm is efficient in real time optimal use of flight level in air traffic control.