The paper proposes an original method for time-domain simulation of MIMO fractional systems based on their rational approximation using Oustaloup's method. Both commensurate and incommensurate fractional systems are tackled. The main contribution is to propose straightforward formulae allowing to approximate a pseudo-state-space representation of a fractional system with a state-space representation of a rational one whatever the dimension of the original system and whatever the number of poles and zeros used in the approximation.