Surface modification of magnetite by 3-aminopropyltriethoxysilane coupling agent (3APTES) at different percentages (5, 10, and 20 wt%) with variable treatment times (3, 5, and 7 h) was performed. The effects of these two variables on the properties of magnetite-filled epoxy thin-film composite were investigated. Treated composites exhibited higher saturation magnetization, storage modulus and T (g) than untreated composites. Increasing 3APTES amount associated with increasing treatment time improved both the interfacial adhesion of magnetite filler matrix and the properties of treated composites. However, using a comparison system at a fixed treatment time, the system treated with a higher concentration of 3APTES exhibited lower saturation magnetization than that treated with a lower concentration. This result can be attributed to the thick coating on the filler surface that acted as a non-magnetic mass to the total sample volume and subsequently reduced the magnetization efficiency.