This paper considers power allocation across MIMO channels for transmitting two different Classes of Service. MIMO provides multiple parallel channels and it transmits different types of services separately because of different Quality of Service requirements. A high priority class of service named as Constant Bit Rate Traffic has limited delay constraints. So priority is given to this traffic in power allocation and then remaining power is allocated to other services i.e. here Best Effort Traffic based on water-filling algorithm. Analytical expressions are presented to assess the outage probability in terms of average access time and the effect of channel estimation error in throughput is also studied. Numerical results and Analysis are also provided to evaluate the Performance indices like outage probability, throughput and probability of access of both classes of service by optimally allocating the power. The Analysis is carried out for uncorrelated Rayleigh-fading channels with and without interference.