In earlier studies,(1,2) we have shown that in MBTS-amidinothiourea and TMTD-amidinothiourea binary systems of rubber vulcanization, amidinothiourea functions effectively as a secondary accelerator and improves the accelerator activity of these systems. These secondary accelerators enhance the physical properties of the vulcanizates and show better cure characteristics of the mixes compared to the reference systems studied. In this paper, we present the results of our study on the effect of various fillers in the sulfur vulcanization of natural rubber using amidinothiourea as the secondary accelerator. We have taken MBTS-amidinothiourea and TMTD-amidinothiourea binary accelerator systems and vulcanizates were prepared with carbon black, precipitated silica, and china clay as fillers. Different compositions of amidinothiourea were used in various mixes using standard recipes. Reference mixes were also studied. Curing characteristics of the mixes and various physical properties of the vulcanizates were evaluated. Optimum dosage of amidinothiourea required has also been derived. Among the different secondary accelerators tried including amidinothiourea, it may be stated generally that the more nucleophilic the secondary accelerator, the less the optimum cure times. This fact points to a nucleophilic reaction mechanism in the systems under review. In the evaluation of tensile and other physical properties of the vulcanizates, some of the systems containing amidinothiourea show better properties while others give comparable values with the reference mixes. Estimation of crosslink density also supports the above conclusion.