Two Garvey-Kelson mass relations are found to be directly related to the n-p interaction. In the case of the same nuclear core, the neutron-proton interaction energy between two neutrons and one proton ∈2n-1p is experimentally found to be close to that between one neutron and two protons ∈1n-2p, which is equivalent to the first Garvey-Kelson mass relation. The sum of∈2n-1p and ∈1n-2p is close to the n-p interaction energy between two neutrons and two protons ∈2n-2p, which leads to the second Garvey-Kelson mass relation. An explanation of these two n-p interaction relations is presented. It is shown that both of these mass relations hold more accurately when the core is an even-even core.