Several solutions have been proposed for improving the Quality of Service (QoS) in wireless cellular networks, such as Call Admission Control (CAC) and handover strategies. However, none of them considers the usage of different interfaces for different conditions. In this work, we propose a Fuzzy-Based Multi-Interface System (FBMIS), where each node is equipped with two interfaces: the traditional cellular network interface and Mobile Ad hoc Networks (MANET)interface. The proposed FBMIS system is able to switch from cellular to ad-hoc mode and vice versa. We consider four input parameters: Distance Between Nodes (DBN), Node Mobility (NM), Angle between Node and Base station (ANB), and User Request Security (URS). We evaluated the performance of the proposed system by computer simulations using MATLAB. The simulation results show that our system has a good performance.