On providing MPEG-2 video service through the ATM network, it is necessary for network provider to have the ATM QoS objectives in order to guarantee quality of service required by end users at the application layer. In this paper, the degradation of QoS caused by cell losses at application layer is assessed quantitatively by GIQ model considering AAL layer and is evaluated qualitatively by MOS from the viewpoint of the end users in order to analyze the relation between CLR and QoS of MPEG-2 video. From the simulation and empirical results, we know that CLR and $GIQ_{mean}$ for guaranteeing QoS of MOS grade 5(Excellent) are $CLR{le}4{ imes}10^7$ & $GIQ_{mean}{ge}99.4%$ and those for guaranteeing QoS of MOS grade 4(Good) are $CLR{le}2{ imes}10^6$ & $GIQ_{mean}{ge}99.705%$.