This paper presents a new fault injection approach, which is based on a co-operation between a simulator and an emulator. This hybrid approach utilizes the advantages of both simulation-based fault injection as well as physical fault injection to provide a good controllability, observability and also a high speed in the fault injection experiments. To do this, parts of a circuit are simulated while the rest parts of the circuit are emulated. A fault injection tool called FITSEC (Fault Injection Tool based on Simulation and Emulation Cooperation) is developed, which supports the entire process of a system design. This is based on both Verilog and VHDL languages and can be used to inject faults at different levels of abstraction. The experimental results show that this approach can significantly reduce the time needed for executing fault injection campaigns.