The mechanical properties of unidirectional natural fiber-reinforced composites are generally affected by several processing parameters during compression molding. This study investigates the effects of processing temperature, time, and pressure on the tensile and flexural properties of acrylonitrile butadiene styrene reinforced by banana fibers. X-ray CT imaging was employed to find the relationship between the mechanical properties and structure of the processed composite. Besides, the water absorption of composites was observed and the way in which the mechanical properties evolved after water absorption was analyzed. The tensile and flexural properties of the unidirectional banana fiber-reinforced composite were found to be inversely proportional to the porosity. In addition, high-pressure compression molding might result in cracks and floating fibers that would significantly reduce its mechanical properties. The composite with the highest strength, smallest porosity and lowest water absorption was optimally prepared at T = 170 °C, t = 20 min, and P = 100 kg cm–2.