Meso-scale metal phthalocyanice (MPc, M=Cu, Ni, Mn, or Co)-Fe3O4 nanocomposties for EMR fluids have been prepared and well characterized. The results indicate that there exists chemical bonding between MPc and Fe3O4 nanoparitcles, and their strength of chempared to Fe2O4 nanoparticles, MPc-Fe2O4 nanocomposites have superior ability to nnti-oxidation and thermal stability, and show almost the same saturation magnetic strength, but much less coercive force, All MPc-Fe3O4 nanocompostites have irregular spherical shape with D of 0.2～0.4μm. The EMR fiulid containing MPc-Fe3O4 nanocomposites in chlorinated paraffin oil have excellent stability and redispersibity. They show reversible significant field-induced shear stress. which increase linearly with the incement of electric field strength (E) or H and which are different for MPc-Fe3O4 nanocomposites containing various metal elements, when E or H is applied respectively. Furthermore, field-induced shear stress shows obvious synergetic effect when E and H are applied simultanceously.