In this study, an innovator, simple, and economical method named roller pre‐stretching is introduced to fabricate polymer‐based composite materials. Stretching technique was applied during vacuum infusion process to enhance the linearity of fibers in the unidirectional composite materials. An increasing linearity proportional together with increasing pre‐stretching was observed on scanning electron microscope (SEM) images. The determined experimental pre‐stretch values were 0, 1.8, 2.7, 4.1, 6.2, and 9.1 MPa. The effect of this method was evaluated under the favor of tensile and three‐point bending tests. Furthermore, volume fractions of the fibers were investigated. The experimental results indicated that the roller pre‐stretching method significantly increased ultimate tensile stress of the composites up to 39% and flexural stress of the composites up to 20%. In addition, stretching fibers over 4.1 MPa did not bring about any improvement on composites in terms of their mechanical properties. POLYM. COMPOS., 40:489–495, 2019. © 2017 Society of Plastics Engineers