Novel hybrid alginate hydrogel beads with shells of porous CaCO3 microparticles were fabricated by templating water-in-oil emulsion and subsequent in situ gelation. Porous CaCO3 microparticles were self-assembled at interfaces of water-in-oil emulsion. Water droplets containing alginate in the emulsion were subsequently in situ gelated by Ca2+ released from CaCO3 through decreasing pH with slow hydrolysis of d-glucono-delta-lactone (GDL). The resulting hybrid beads with alginate gel cores and shells of porous CaCO3 microparticles were called colloidosomes. The packed density of CaCO3 microparticles in the shell increased with increasing the ratio of the CaCO3 microparticle weight to the water phase volume Mp/Vw and decreased with addition of NaCl into water. The size of the produced colloidosome beads was independent of Mp/Vw. Increasing the volume fraction of water Phi w to 0.5, some colloidosome beads deformed to nonspheral shape and even broken. Brilliant blue (BB) as a drug model was loaded into the colloidosome beads by being dissolved in the alginate aqueous solution before gelation. The BB release from the colloidosome beads was slowed down because of the formation of the shells of CaCO3 microparticles. The colloidosome beads may find applications as delivery vehicles for drugs, cosmetics, food supplements and living cell.