The strengthening mechanism due to copper (Cu) particles was discussed in terms of the interaction between dislocation and Cu particles in aged Fe-Cu alloys. Since Cu particles are softer than the iron matrix, its interaction with dislocation is different from that with the Orowan mechanism. The moving dislocations can cut the soft Cu particles and pass through them when the bowing angle reaches some critical value (θ_c), and the precipitation strengthening due to Cu particles is expressed as a function of mean particle spacing (λ) and the θ_c (π/2 gives Orowan stress). The θ_c increased with increasing the size of Cu particles and reached π/2 when the Cu particle size became 70 nm. This means that the precipitation strengthening due to Cu particles is dependent on not only λ but also the Cu particle size, and 70 nm is the minimum Cu particle size for obtaining the Orowan stress.