Most partial altervalent/aliovalent substitutions for Mg or B in MgB2 studied to date depress the superconducting transition temperature (T-c) and, at higher replacements, completely suppress superconductivity of MgB2. The diminution and loss of superconductivity in MgB2 arise from the subtle interplay between the competing/cooperating effects of the electronic and lattice structural variations, which are induced by the different charge and atomic radii of the substiments. Here, we experimentally discriminate lattice structural effects from electronic contributions to superconductivity by exploiting the nanosize dependence of the lattice structure to modify structural parameters without resorting to chemical doping. It is found that the superconductivity of MgB2 is extremely sensitive to lattice parameter variation, such that contraction of Mg-Mg bond dramatically depresses T-c and eventually results in the loss of superconductivity as the average coordination of Mg to B falls from 12 to 8 due to the introduction of B vacancies for nanocrystalline MgB2 of 2.5 nm diameter.