The rare earth lanthanum oxide doped molybdenum alloys containing different volume fraction lanthanum oxide particles were prepared by powder metallurgical processing. The microstructure and mechanical properties of the alloys were examined. It is found that the alloy has the fine lanthanum oxide particles and fine grain as well as high yield strength. The strengthening mechanism of the alloy is analyzed and explained in three aspects: matrix strengthening, fine grain size strengthening yielding Hall-Petch relationship, and fine particles strengthening according to the Orowan model. The quantitative relationships between the yield strengths of the alloys and the sizes, volume fraction of lanthanum oxide particles and the grain sizes were also presented.