Abstract: It is found that the relation among single bond length (rHC, rAB), covalent radius (rH,rC,rA,rB) and percent ionic character (i) is quite accurately given by the expressionsrHC=rH+rC- (0.18i-1/2/rc +0.05) for hydrogen bond (H-C) in hydride and rAB=rA +rB -3-1 ir1/2/A r-1/2/B for bond (A-B) in other compound, in which the electronegativity of atom B is greater than that of atom A. From the calculated bond lengths the reduced partition function ratios of various molecular species are estimated.