Disclosed are nanoparticles that are introduced into cells and express a specific protein and a manufacturing method thereof. More particularly, the present invention relates to mRNA nanoparticles, which increase the expression of a specific protein capable of stimulating the cellular immune system to induce cellular immune responses and are thus applicable to treat a variety of diseases, do not require passage across the nuclear envelope because a desired gene is delivered not as plasmid DNA itself but in the form of mRNA, thus improving the efficiency of protein expression, and the nanoparticles are generated through a one-step process with a relatively small amount of plasmid DNA via rolling circle transcription (RCT), thereby providing a simple and economical process for gene delivery. The present invention is also concerned with such mRNA nanoparticles.