AbstractPhysical chemical properties of cupuassu fat were modified by dry fractionation. Stearin and olein fractions were obtained at 29, 26, and 24 °C. Polymorphic behavior of unfractionated cupuassu fat (UCF) and its fractions were studied in situ by small‐angle (SAXS) and wide‐angle (WAXS) X‐ray scattering using synchrotron light. Polymorphic transitions were followed in real time tempering samples with a thermal cycle. For UCF, the main polymorphic form crystallized under selected conditions was the β’2. α and β’1‐forms appeared in trace amounts. β2‐form was obtained after storage at 25 °C for 3 months. Stearins obtained at 26 (S‐26) and 24 °C (S‐24) showed a similar polymorphic behavior. However, S‐26 with improved physical properties might be more suitable for chocolate production or as a trans‐fat alternative than UCF. Stearin fraction obtained at 29 °C (S‐29) had a complex polymorphic behavior. The α‐form was the first polymorphic form detected followed by β’2‐form. There was a polymorphic transition from α to β’1‐form but no transition between β’‐forms. They were independent to each other showing fractionation in two different solid solutions. Increased contents of the triacylglycerols (TAG) SOA and SOB together with lower contents of SOO compared to UCF led to co‐crystallization because there was no complete compatibility among all TAG present in S‐29. β1‐form crystallized after storage forming crystals with a double‐layer arrangement and a characteristic morphology. This form could be useful for accelerating crystallization process in melted liquid systems.