In this paper electromagnetic transient (EMT) simulation of large scale power systems using graphics processing unit (GPU) based computing is demonstrated. As the size of power system networks increases, the simulation time using conventional central processing units (CPUs) based simulation increases drastically. This paper proposes a hybrid CPU-GPU environment for fast large scale power systems simulation. In this scheme the GPU is mainly deployed to perform the computationally intensive part of the simulation in parallel on its built-in multiple processing cores, and the CPU is assigned for other sequential jobs like flow control of the simulation and storing output data, etc. The GPU-based approach is used to simulate a network with 900 Buses, and it is shown that the CPU-GPU based implementation is 70 times faster than the conventional CPU-based implementations.