We analytically solve the Non-Markovian single electron spin dynamics due to hyperfine interaction with surrounding nuclei in a quantum dot. We use the equation-of-motion method assisted with a large field expansion, and find that virtual nuclear spin flip-flops mediated by the electron contribute significantly to a complete decoherence of transverse electron spin correlation function. Our results show that a 90 electron spin T_2 time by almost two orders of magnitude. In the long time limit,

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... e electron spin correlation function has a non-exponential 1/t^2 decay in the presence of both polarized and unpolarized nuclei.