In this article, we proposed a facile one-step synthesis of Fe 3 O 4 nanoparticles of different shapes and sizes by co-precipitation of FeCl 2 with piperidine. A careful investigation of TEM micrographs shows that the shape and size of nanoparticles can be tuned by varying the molarity of piperidine. XRD patterns match the standard phase of the spinal structure of Fe 3 O 4 which confirms the formation of Fe 3 O 4 nanoparticles. Transmission electron microscopy reveals that molar concentration of FeCl 2 solution plays a significant role in determining the shape and size of Fe 3 O 4 nanoparticles. Changes in the shape and sizes of Fe 3 O 4 nanoparticles which are influenced by the molar concentration of FeCl 2 can easily be explained with the help of surface free energy minimization principle. Further, to study the magnetic behavior of synthesized Fe 3 O 4 nanoparticles, magnetization vs. magnetic field (M-H) and magnetization vs. temperature (M-T) measurements were carried out by using Physical Property Measurement System (PPMS). These results show systematic changes in various magnetic parameters like remanent magnetization (Mr), saturation magnetization (Ms), coercivity (Hc), and blocking temperature ( T B ) with shapes and sizes of Fe 3 O 4 . These variations of magnetic properties of different shaped Fe 3 O 4 nanoparticles can be explained with surface effect and finite size effect.