In this thesis, we consider black holes as particle accelerators. We investigate that whether or not black holes are source of high energy cosmological particles. At first we concentrate on none-rotating and rotating four dimensional black holes. After calculating the center of mass energy of two colliding particles in the vicinity of event horizon, we understand that extremely and nearly extremely rotating black holes can accelerate particles to arbitrary high energy. It is an important issue to study nearly extremely black holes, because the existence of such black holes is strongly suggested by astrophysical observations. Next concentrating on higher and lower dimensional black holes, we obtain geodesic equations. In similar way, we calculate center of mass energy of two colliding particles and understand that extremely rotating black holes can accelerate particles to arbitrary high energy. Rotating black holes can accelerate particles to arbitrarily high energy if the angular momentum of the particle is fine-tuned to some critical value. To find critical value we should set effective potential and its first derivation zero. The r geodesic equation for a particle is reduced to a simple one-dimensional potential problem.
