Knowledge of various amaranth flour (AF) particle sizes influence added in different amounts in refined wheat flour (WF) seems necessary to properly design systems with expected rheological properties. The study aimed to assess the effects of factors, particle size and level of AF added in WF on Falling number index (FN) and dough rheological properties described by Mixolab in a systematic investigation based on design of experiment. The level of AF added in WF significantly influenced the FN values in the linear and quadratic terms (p< 0.05), revealing a decrease with the level increase. Particle size played a significant role in increasing water absorption in linear and quadratic terms (p< 0.05), whereas only the linear term has a significant effect (p< 0.005) on dough development time and stability. Particle size increase and the interaction between particle size and AF significantly increased torques C2 and C3. AF level increase in WF significantly decreased the torques C4 and C5. The optimization results highlighted that the most suitable combination would be the composite flour with 9.74% AF of 280 μm particle size incorporated in WF. The proposed approach and the information resulted can be relevant for industrial applications, helping to develop products with amaranth with optimum rheological properties, in order to scale-up the production.