In this paper, we define the transition semi-wave solution (c.f. Definition 1.1) of the following reaction diffusion equation with free boundariesIn the homogeneous case, i.e., f (t, x, u) = f (u), under the hypothesiswe prove that the semi-wave connecting 1 and 0 of (1) is unique provided it exists. Furthermore, we prove that any bounded transition semi-wave connecting 1 and 0 is exactly the semi-wave.In the cases where f is KPP-Fisher type and almost periodic in time (space), i.e., f (t, x, u) = u(c(t) − u) (resp. u(a(x) − u)) with c(t) (resp. a(x)) being almost periodic, applying totally different method, we also prove any bounded transition semiwave connecting the unique almost periodic positive solution of u t = u(c(t) − u) (resp. u xx + u(a(x) − u) = 0) and 0 is exactly the unique almost periodic semi-wave of (1). Finally, we provide an example of the heterogeneous equation to show the existence of the transition semi-wave without any global mean speeds. transition semi-waves, propagation problems, free boundary problems, reaction diffusion equations 1 arXiv:1809.08551v1 [math.AP]