Nano magnetite particles have been prepared by two step reaction consisting of urea hydrolysis and ammonia addition at certain ranges of pH. Three different concentrations of aqueous solution of ferric ($Fe^{3+}$) and ferrous ($Fe^{2+}$) chloride (0.3 M-0.6 M, and 0.9 M) were mixed with 4 M urea solution and heated to induce the urea hydrolysis. Upon reaching at a certain pre-determined pH (around 4.7), 1 M ammonia solution were poured into the heated reaction vessels. In order to understand the relationship between the concentration of the starting solution and the final size of magnetite, in-situ pH measurements and quenching experiments were simultaneous conducted. The changes in the concentration of starting solution resulted in the difference of the threshold time for pH uprise, from I hour to 3 hours, during which the akaganeite (${eta}$-FeOOH) particles nucleated and grew. Through the quenching experiment, it was confirmed that controlling the size of ${eta}$-FeOOH and the attaining a proper driving force for the reaction of ${eta}$-FeOOH and $Fe^{2+}$ ion to give $Fe_3O_4$ are important process variables for the synthesis of uniform magnetite nanoparticles.