  Motivated by the recent experimental work (A. Widera, \textit{et al}, Phys.
Rev. Lett. 95, 19045), we study the collisional spin dynamics of two spin-2 $%
^{87}$Rb atoms confined in a deep optical lattice. When the system is
initialized as $|0,0>$, three different two-particle Zeeman states are involved
in the time evolution due to the conservation of magnetization. For a large
magnetic field $B>0.8$ Guass, the spin coherent dynamics reduces to a Rabi-like
oscillation between the states $|0,0>$ and $% |1,-1>$. However, under a small
magnetic field, a general three-level coherent oscillation displays. In
particular, around a critical magnetic field $B_{c}\simeq 0.48$ Guass, the
probability in the Zeeman states $% |2,-2> $ exhibits a novel quantum beat
phenomenon, ready to be confirmed in future experiments.
