In this paper, we report on the magnetic properties of Fe304 nanoparticles with different grain sizes under differentpressures. In all the samples, the saturated magnetization Ms shows a linear decrease with increasing pressure. Thethickness of the magnetic dead layer on the nanoparticle surface under different pressures was roughly estimated, whichalso increases with increasing pressure. The transport measurements of the nanoparticle Fe304 compacts show thatthe low-field magnetoresistance (MR) value is insensitive to the grain size in a wide temperature range; however, thehigh-field MR value is dependent on grain size, especially at low temperatures. These experimental results can beattributed to the different surface states of the nanoparticles.