Abstract: Iron/polyacrylate composite particles were synthesized through inversed emulsion polymerization technique. The particles were then dispersed in silicone oil containing different additives to form electro and magneto rheological(ER and MR) fluids. Studying the influence of the additive on electro and magneto rheological effects, reveals that the fluid exhibits strong MR effects which do not change obviously with the type and amount of the additive, such as Span 80, MOA 3 and KH 550, while ER effects of the fluid change remarkably and differently in the same condition. The ER effect of the fluid decreases with increasing Span 80 content, while as for KH 550 it decreases in a much smaller degree. MOA 3 increases and then decreases the ER effect, leading to a maximum excess shear stress at the content of 1%~2%. This ER and MR effect difference stems from the different mechanism of the particle polarization and the interaction between the polarized particles under the electric and magnetic field.