Silicon nitride hollow quasi-spheres (SNHQSs) are prepared via a two-step method: producing micrometer-sized agglomerated granules with -Si3N4, and sintering additives of nano-alumina and nano-yttria by spray-drying and then hollowing the spray-dried granules (SDGs) by radio frequency thermal plasma sintering. Five kinds of slurry with different content of sintering additives are prepared for spray-drying, and the attained five kinds of SDGs have similar morphology and properties. However, different SDGs generate diverse hollow structures via plasma sintering. The cavity formation mechanism of the SNHQSs is deeply investigated. In addition, the obtained SNHQSs possess low apparent density (0.371-0.481g/mL), high compressive strength (up to 50MPa), and good thermal stability (up to 1600 degrees C), which will enable their promising applications in porous ceramics.