The microRNA(miRNA)-34a is an important regulator of tumor suppression. It controls the expression of several target proteins involved in cell cycle, differentiation and apoptosis, and antagonizes processes that are necessary for basic cancer cell viability as well as cancer stemness, metastasis, and chemoresistance. It is downregulated in numerous cancer types, including prostatic cancer, and inhibits malignant growth by repressing genes involved in various oncogenic signaling pathways. Given the anti-oncogenic activity of miR-34a, here we proved the substantial benefits of a new therapeutic concept based on nanotechnology delivery of miRNA mimics. In order to monitor the miRNA-34a replacement, we used a fluorescent nanodiamond particles (FND) system with linked miRNA-34a mimic, which was delivered to PC3 and DU145 prostatic cancer cell lines. We used functionalized nanodiamonds coated with polyethylenimine to transfer miRNA-34a into PC3 and DU145 prostatic cancer cell lines and we measured the zeta-potential of these complexes before using them for in vitro experiments. A replacement of miRNA-34 was observed by monitoring levels of miRNA-34 via real-time PCR. Moreover, our in vitro experiments demonstrated that miRNA-34a replacement, using this FND delivery system, decreased viability and induced apoptosis in prostatic cancer cell lines. Our findings suggest the replacement of oncosuppressor miRNA-34a provides an effective strategy for cancer therapy and the FND-based delivery systems seems to be an excellent strategy for a safe and effective targeting of the tumor.