The anisotropic thermal expansion and crystal structure of K2NiF4-type CaErAlO4 have been investigated by neutron diffraction from 298 to 1473 K. The average thermal expansion coefficient (TEC) along the c axis alpha(c), is larger than that along the a axis, mainly due to the larger Al-(apical oxygen O2) TEC compared to the Al-(equatorial O1) TEC. The larger Al-O2 TEC is attributable to the weaker Al-O2 bond. Contrary to the literature, the mean TEC of K2NiF4-type CaErAlO4 is larger than that of perovskite-type ErAlO3.