Based on ab initio theory and nonequilibrium Green's function method, rectification in diblock pyrimidinyl-phenyl molecular diodes with various molecular lengths is investigated theoretically. The results demonstrate two opposite rectifying directions depending on the molecular length. Molecular orbital analysis via molecular projected self-consistent Hamiltonian reveals two competitive rectification mechanisms, asymmetric molecular level shift and asymmetric evolution of electronic wave functions under biases, determining the rectifying direction. A critical molecular length is predicted where the dominant mechanism is exchanged. The ab initio calculations display an intrinsic inversion of rectification caused by molecular length in pyrimidinyl-phenyl diodes, which provides a way to modulate rectifying behaviors. (C) 2013 Elsevier B. V. All rights reserved.