In this paper, we prove sharp gradient estimates for a positive solution to the heat equation u t = ∆u + au log u in complete noncompact Riemannian manifolds. As its application, we show that if u is a positive solution of the equation u t = ∆u and log u is of sublinear growth in both spatial and time directions then u must be constant. This gradient estimate is sharp since it is well-known that u(x, t) = e x+t satisfying u t = ∆u. We also emphasize that our results are better than those given by Jiang ([Jia16]), Souplet-Zhang ([SZ06]), Wu ([Wu15, Wu17]), and others.