In this paper, the effects of particles size of Magnetorheological Carbonyl iron powder on damping force and energy dissipation capacity for a Magnetorheological double ended type damper is investigated experimentally. Despite of the considerable researches on the effects of particles size on the viscosity of Magnetorheological fluids, sedimentation of fluids and electromagnetic field intensity in damper, there is no a published work about the effects of iron particles size on the damping force amplitude and energy dissipation capacity of double-ended Magnetorheological damper. Therefore, in the present research, two different Magnetorheological fluids were prepared with the same volumetric percentage of % 35 from two different sizes of Iron particles i.e. 40 µm and 63µm and filled into a double ended type damper. The double-ended damper had three electric coils and was tested in different frequencies, different electric currents and 15 mm displacement stroke. The effects of Magnetorheological fluid particles on produced damping force and energy dissipation capacity were analyzed by extracting force-displacement and force-time curves from experiments. The results showed that the maximum amplitude of damping force is increased with increasing the applied electric current on the damper and the amount of this force for fluid with 63µm particles size is slightly higher than that for the fluid with 40µm particles size. However, the energy dissipation capacity of the investigated damper in all excitation frequencies with the all applied electrical currents for fluid with 63µm particles size was considerably higher than that for fluid with 40µm particles size.