Low-density cobalt-phthalocyanine (Co-Pc)/Fe nanocomposite particles with both adjustable electric and magnetic properties were synthesized using one-step thermolysis. The nanocomposite particles were fully characterized by Fourier transform infrared spectroscopy, X-ray diffraction analysis, scanning electron microscopy, high-resolution electron microscopy, and thermogravimetric-differential thermal analysis. The magnetic hysteresis loop and microwave electromagnetic parameters of the nanocomposite particles were measured. The magnetorheological (MR) properties of the MR suspensions based on the nanocomposite particles dispersed in methyl silicone oil were investigated as functions of weight percent of particles, magnetic field strength (H), temperature (T), and shear rate (y). The results indicated that the Co-Pc/Fe nanocomposite consisted of micrometer-sized regular spheroids with hundreds of thousands of Co-Pc-coated alpha-Fe nanoparticles on the inside and Co-Pc layers on the surface of the spheroids. It showed good characteristics of antioxidation and high magnetic susceptibility. For nanocomposites containing 82.7 wt % Fe, with increasing microwave frequency, the complex permittivity (E,) gradually reduced while the real part minimum and imaginary part maximum of the complex permeability (p,) occurred. In comparison with conventional carbonyl iron powders, the density of the nanocomposite particles was much lower, and er decreased significantly while mu(r) hardly changed. Magnetic field induced shear stress of the MR suspension based on nanocomposites with 89.7 wt % Fe was basically independent of T and y but increased abruptly with increasing H. With increasing T, response time of the MR suspension to an external magnetic field seemed to decrease. The results suggested that the synthesized Co-Pc/Fe nanocomposite particles with different compositions could be utilized as an excellent microwave absorber and as dispersed particles of high-performance MR suspensions.