Phase transitions and structures of organic ultra-thin films have been investigated from temperature dependence measurements of X-ray diffraction (XRD) and differential scanning calorimetry (DSC) using both Langmuir-Blodgett (LB) films and a Mn-arachidate salt powder. DSC revealed that, unlike pure arachidic acid, the Mn-arachidate powder showed two-phase transitions, i.e., the main transition and a new transition or pretransition. XRD of the LB films confirmed the two-phase transitions, and elucidated three phases, namely, a lamellar phase with a certain tilt, a lamellar phase without tilt, and an ordered fluid phase with melted alkyl-chains. These findings indicate that the phase behavior plays a key role in the fabrication mechanics of LB films of transition metal-fatty acid salt, such as the temperature control during the monolayer formation and the annealing processes.