aCompared to chemical pulp, mechanical and chemi-mechanical pulps (CMP) are limited in regards to the manufacturing of high quality paper. Chemical additives are an effective way to enhance the properties of paper; however, the effectiveness depends on the additive type and dosage. The utilization of cationized natural polymers has been shown to offer a promising solution. In this study, softwood long fiber (SLF) and CMP were cationized by 3-chloro-2-hydroxypropyl-trimethylammoniumchloride (CHPTAC), and the effects of cationization on the properties of CMP pulp were studied. Cationization was characterized by FTIR and the nitrogen content, and its effect on the CMP properties was evaluated through mechanical tests and fines retention. Cationization at low and moderate levels and in higher mixing rates improved the mechanical properties of CMP. Compared to cationized CMP, the addition of cationized SLF (CLF) improved the strength and fines retention properties. The CLF application to CMP at a CHPTAC dosage of 5% increased the tensile, burst, and tear strengths by 66.4%, 100%, and 3.6%, respectively. The cationized SLF increased the fines retention by 12.7%.