It was reported that layered manganite $La_{1.2}Sr_{1.8}Mn_2O_7$(327-phase) shows magnetic phase transition at $T_c$ = 128K, and couple of more traces of transitions over $T_c$, which are believed to be due to intergrowth of $(La,Sr)MnO_3$(113-phase). Mn NMR spectrum clearly shows two separate peaks coming from the 327-phase and 113-phase. The signal intensity generated by the 327-phase does not vanish above $T_c$, meaning that the ferromagnetic metallic clusters of 327-phase exists above $T_c$. Most abnormal feature of this material is that La NMR signal intensity decreases very slowly as the temperature increases from below $T_c$ to above $T_c$, where the decay time is about 30 minute. Such anormalous behaviors can be explained by the cluster glass model where the interactions between the ferromagnetic clusters of the 113-phase mediated by the 327-phase are frustrated
