<p>This study examines the impact of polyethylene terephthalate (PET) fibers on major mechanical properties of cement mortar including toughness and modulus of rupture (MOR). For this purpose, various mixes were designed and PET fibers were incorporated into mortar mixes at two dosage levels (0.5% and 1% by volume), along with two other varying parameters i.e. water-to-binder (w/b) ratios, and superplasticizer (SP) ratio to adjust the workability of the cement mixtures. The key mechanical characteristics of hardened cement composite including strength, toughness, and MOR were evaluated and a comprehensive parametric study was implemented. Besides, correlation analyses of each pair of the mechanical properties were performed and the coefficients of determination were determined. The experimental outcomes indicated that the optimum fiber volumes for compressive toughness and flexural toughness were 0.5% and 1%, respectively. While lower w/b ratios such as w/b = 0.35 resulted in higher strength properties, improved toughness was noted in higher rations. RSM analysis implemented showed that the optimum MOR can be achieved at the lowest level of w/b and middle levels of SP and Fiber%. Correlation analyses results demonstrated a strong correlation between compressive strength-toughness and compressive strength-MOR with R² = 0.90 and R² = 0.96, respectively. However, lower correlations were observed for flexural strength-toughness and flexural toughness-MOR with similar R² = 0.75.</p>