Spray pyrolysis is a droplet-particle conversion process comprising of the evaporation of solvents, precipitation of the solutes, decomposition of the solutes, sintering oi the formed phases, etc. Purposive control of the reactions within every droplet or particle makes it possible to control the structure, phase composition and shape of the prepared particles. By introducing reactions into every droplets or precisely controlling the intraparticle reactions, sintering and phase separation, direct preparations of either dense or hollow, single-crystal or layer-structured particles were achieved by spray pyrolysis.