Nano particles of Fe3O4 with the range of 30 and 60 nm were synthesized by using chemical coprecipitation method. Particle size effects according to magnetic properties were investigated together with the constant reaction and crystallization temperature. According to calculations peaks were indexed as cubic Fe3O4 for all samples. The hysteresis loops shows superparamagnetic behavior. It was found that when Fe2+/Fe3+ ratio increase to 50% the particle size increases approximately one fold. We realized that chemical coprecipitation method is suitable for biomedical applications of Fe3O4 nano particles.