Abstract: The microwave tranamission characteristics of a ferrofluid (the carrier liquid being hydrocarbon and the magnetic particles being magnetite) magnetized with the biasing field H, have been measured. The phenomenon of ferro-magnetic resonance absorption in the ferrofluid has been observed. When the ferrofluid is biased far from the resonance field, low-loss transmission and non-reciprocal transmission, i.e., non-reciprocal attenuation and shift in the waveguide have been obtained respectively.