This thesis deals with the optimal control of traffic flow on urban traffic networks. For this, a nonlinear discrete-time model of urban traffic network is first suggested, which can describe the behaviours of traffic flows such as oversaturatedness and/or undersaturatedness. Then an optimal control problem is formulated with the proposed model. To solve the optimal control problem, a hierarchical optimization technique is also proposed whigh is based upon a prediction-type two-level method of Hirvonen and Hakkala.