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The magnetic relaxation properties of two derivatives of Mn12 single-molecule magnets (SMMs), Mn12-Ac and Mn12-tBuAc, are investigated and compared in this paper. By a simplified "hole digging" method, the hyperfine fields turn out to be very similar in these two compounds (39±5 mT for Mn12-Ac at −3.55 T and 32±5 mT for Mn12-tBuAc at −3.2 T). The small difference of hyperfine fields received by the same magnetic cores Mn12O12 in these two SMMs shows that the hyperfine field is not the main source of the large difference of magnetic relaxation rates between Mn12-Ac and Mn12-tBuAc. The intentional inclined-field experiments show that a larger transverse component of the dipolar field H dip,x in Mn12-Ac causes a faster magnetic relaxation rate only when the transverse component of the external applied field Hx exceeds 0.5 T. On the other hand, a larger longitudinal component of the dipolar field H dip,z pushes more molecules out of tunneling, then decreases the relaxation rate. Therefore, the faster magnetic relaxation rates in Mn12-tBuAc result from its smaller H dip,z . Furthermore, it is found that the hereditable characteristic of the "hole digging" effect in these two SMMs is relative to the dipolar field distribution.