Multi-subband ensemble Monte Carlo simulators (MSB-EMC) are essential in semiconductor device modelling in order to study confinement effects when devices are scaled to gate lengths and silicon thicknesses approaching nanometre dimensions. To consider these effects it is necessary to solve the Schrödinger equation in the confinement direction, which has a high computational cost. In this paper we propose the parallelisation and optimisation of a 2D MSB-EMC simulator. The Open Multi-Processing (OpenMP) application program interface (API) is employed for the parallelisation and the simulations are performed using a multi-core machine. Numerical results show a speed-up of 7.40 when eight cores are used with a parallel efficiency higher than 90%.