Magnetorheological fluid (MRF) is a kind of promising smart functional material with significant magnetorheological (MR) performance. MRFs are typically fabricated by mixing magnetizable microparticles into a base oil. As the most important part of MRFs, magnetizable carbonyl iron (CI) particles show an obvious impact on the performance of MRFs. In this work, MRFs based on six types of CI particles were fabricated, and their MR performance was tested under both rotational and oscillatory modes by a parallel plate rheometer coupled with an MR device. The results indicate that MRFs based on big particles with high iron content present more significant MR performance in both rotational and oscillatory modes. Conversely, small particles were likely to form weaker structures that displayed lower shear stress and storage modulus. Meanwhile, silicon dioxide–coated CI particles led to decreased shear stress and storage modulus but dilated the linear viscoelastic range of MRFs. MRFs based on phosphate-modified CI particles presented a high storage modulus but low shear stress. Moreover, the normal force test results also suggest that big particles with high iron content are likely to form stronger microstructures that macroscopically present relatively higher normal forces. The suspension stability of MRFs was measured by direct observation. The results indicate that MRFs based on surface-modified and small particles have better suspension stability.