This study reports the first morphology and crystalline structure details of iron oxide nanoparticles in a comprehensive manner. A series of iron oxide nanoparticles were synthesized in 1-octadecene from iron(III) acetylacetonate with the aid of oleic acid surfactant and then followed by post thermal processes. Quantitative small and wide angle X-ray scattering analyses using synchrotron radiation sources were performed together with electron microscopy, infrared spectroscopy and thermogravimery, providing morphology and crystalline structure details. Larger size of nanoparticles are synthesized by higher loading of the surfactant. Prolate ellipsoidal nanoparticles, rather than spherical particles, are always synthesized in single unimodal and narrow size distribution. The individual particles are composed of core, core-shell interface, shell, and shell-surfactant interface, regardless of the sizes. Magnetite-like crystalline phases are predominant. In addition, wuestite-like crystalline phases are discernible as minor components. For a given particle, the size and distribution are varied very little by the post thermal proccesses. Nevertheless, the other morphology characteristics, as well as the crystalline phases are significantly influenced through the post thermal process with a mixture of nitrogen and oxygen. In particular, the core part is thickened, the density gap between the core and the shell is reduced, and ferrimagnetic magnetite-like crystallites are enlarged and more populated. Paramagnetic wuestite-like crystalline phases are decreased substantially or disappeared completely. These enhanced morphology and crystalline characteristics make great contributions to improve magnetization performances significantly. Overall, this study provides the well-controlled synthetic schemes and morphology/crystalline structure details that are essential for better applications of iron oxide nanoparticles in various advanced fields including biomedicine and nanotechnology.