A series of mononuclear Dy-III compounds, [Dy(tmpd)(3)(4,4'-dmpy)] (1), [Dy(tffb)(3)(4,4'-dmpy)] (2), [Dy(tffb)(3)(5,5'-dmpy)] (3), and [Dy(tmpd)(3)(5,5'-dmpy)] (4) [tmpd = 4,4,4-trifluoro-1-(4-methoxyphenyl)-1,3-butanedione, tffb = 4,4,4-trifluoro-1-(4-fluorophenyl)-1,3-butanedione, 4,4'-dmpy = 4,4'-dimethyl-2,2'-bipyridyl, and 5,5'-dmpy = 5,5'-dimethyl-2,2'-bipyridyl], have been synthesized by modifying beta-diketonate ligands and capping N-donor co-ligands. Dy-III ions in 1-4 possess N2O6 octacoordinated environments. Compounds 1 and 2 exhibit distorted trigonal dodecahedron configurations, while 3 and 4 display distorted square antiprismatic configurations. Systematic investigations of the alternating current measurements indicate the different magnetic relaxation dynamics with energy barriers (U-eff) of 66 K (1, 45 cm(-1)), 189 K, (2, 131 cm(-1)), 115 K (3, 79 cm(-1)), and 205 K (4, 142 cm(-1)). To deeply understand their different magnetic behaviors, the magnetic anisotropies of 1-4 were studied by ab initio calculations. From ab initio calculations, the energies of the first excited state (KD1) are consistent with the experimental U(eff )under zero direct current field. Compound 4 presents the largest U-eff because of the smallest g(X,Y) and mu(QTM) as well as the most strong axial crystal field parameters (CFPs) among compounds 1-4. The M versus H data exhibit butterfly-shaped hysteresis loops at 2 K for 1-4. The different coordination geometries, the magnetic dynamics, the electrostatic repulsion, and CFPs result from the different substituent effects of ligands, including the electronic effect, the steric effect, and the positions of substituted groups.