The emerging field of RNA nanotechnology necessitates creation of functional RNA nanoparticles, but has been limited by particle instability. Previously, it was found the three-way junction (3WJ) of the Phi29 DNA packaging motor pRNA was found to be ultra-stable and assemble in solution without the presence of metal ions. The three-way junction is composed of three short oligo RNA strands and proven to be thermodynamically stable. Here the assembly mechanism, thermodynamic and enzymatic stabilities, and kinetics are examined in order to understand the stability behind this unique motif. Thermodynamic and kinetics studies found that the pRNA 3WJ formed out of three components at a rapid rate creating a single-step three component collision with a lack of dimer intermediate formation while being governed by entropy, instead of the commonly seen enthalpy. Furthermore, the pRNA 3WJ proved to be stable at temperatures above 50 °C, concentrations below 100 pM, and produced a free energy of formation well below other studied RNA structures and motifs. With the high stability and folding efficiency of the pRNA 3WJ, it serves as an ideal platform for multi-branched RNA nanoparticles constructed through bottom-up techniques. RNA nanoparticles were constructed for the specific targeting of prostate cancer cells expressing Prostate Specific Membrane Antigen (PSMA) by receptor mediated endocytosis through the addition of an RNA aptamer; and the delivery of anti-miRNA sequences for gene regulation. The resulting nanoparticles remained stable while showing highly specific binding and entry in PSMA positive cells through cell surface receptor endocytosis. Furthermore, the entry of the nanoparticles allowed for the knockdown of against onco-miRNAs. Nanoparticles harboring antimiRNAs led to the upregulation of tumor suppressor genes, and signaling of apoptotic pathways. These findings display RNA nanotechnology can result in the production of stable nanoparticles and result in the specific treatment of cancers, specifically prostate cancer.