K2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (ac) being higher than that along the a-axis (aa) of LaSrAlO4 [ac = 1.882(4)aa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O-2 a(Al-O-2) being higher than that between Al and equatorial oxygen O1 a(AlO1) [a(Al-O-2) = 2.41(18)a(Al-O1)]. The higher a(AlO2) is attributed to the AlO2 bond being longer and weaker than the AlO1 bond. Thus, the minimum electron density and bond valence of the AlO2 bond are lower than those of the AlO1 bond. For Sr2TiO4, the TiO2 interatomic distance, d(Ti-O-2), is equal to that of TiO1, d(TiO1) [d(Ti-O-2) = 1.0194(15)d(Ti-O-1)], relative to LaSrAlO4 [d(AlO2) = 1.0932(9)d(Al-O-1)]. Therefore, the bond valence and minimum electron density of the Ti-O-2 bond are nearly equal to those of the Ti-O-1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A(2)BO(4), is strongly influenced by the anisotropy of BO chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B-O-2)/d(B-O-1) of A(2)(2.5+)B(3+)O(4) compared with A(22+)B(4+)O(4), A(22.5+)B(3+)O(4) compounds have higher a(B-O-2), and A(22+)B(4+)O(4) materials exhibit smaller a(B-O-2), leading to the anisotropic thermal expansion of A(22.5+)B(3+)O(4) and isotropic thermal expansion of A(22+)B(4+)O(4). The true thermal expansion without the chemical expansion of A(2)BO(4) is higher than that of ABO(3) with a similar composition.