The given research revealed that the size of Fe₃O₄ magnetic nanoparticles (MNPs) could be controlled by varying the pre-mixing conditions in the solvothermal method. Scanning electron microscopy (SEM) showed that the size of the MNPs gradually increased with increasing the initial temperature at which reaction components were mixed while the reaction component's mixing time was kept constant. The smallest sized MNPs were achieved among the five treatments (25, 50, 75, 100, and 125 °C) when reaction components were mixed at 25 °C, while the larger sized MNPs were synthesized among the five treatments when reaction components were mixed at 125 °C. Then, Stöber method was followed for coating silica layer onto the MNPs. However, ammonium hydroxide was replaced with potassium hydroxide as a catalyst, which significantly increased the speed of silica coating onto MNPs. The Fourier transform infrared (FTIR) spectrometer revealed that the MNPs were successfully covered with silica in five minutes. FTIR spectra exhibited a peak about 1088.8 cm-1, which belonged to the asymmetry stretching vibration of Si-O-Si. Transmission electronic microscopy (TEM) analysis was conducted to confirm the presence of silica layer onto MNPs. Thus, potassium hydroxide was successfully employed as a catalyst for quick silica layer coating onto MNPs. Furthermore, these silica coated MNPs were used to extract high quality nucleic acids from blood sample.