A sequence of $S_n$-representation $V_n$ is called representation stable if after some n, the irreducible decomposition of $V_n$ stabilizes. In particular, Church et al. found that if we fix a and b, then the space of diagonal harmonics $DH_n^{a,b}$ exhibits this behavior. Following this result, we get the dimension of the bigraded space $DH_n^{a,b}$ as a polynomial combinatorially, and improve the stability range. In this talk I will briefly introduce representation stability, the space of diagonal harmonics, and present the idea of the proof of the aforementioned polynomial.