Pineapple leaf fiber (PALF) having an average diameter and length of about 4 μm and 6 mm, respectively, was used as reinforcing element for natural rubber (NR) composites. PALF was coated with different amount of stearic acid at 10, 30 and 50 wt% of PALF. PALF-NR composites containing two levels of PALF at 5 and 10 part per hundred rubber (phr) were prepared in a two roll mill. Mastication times of 2, 4 and 8 min were used. Tensile stress-strain curves and fracture surfaces of both untreated PALF and stearic acid coated PALF (SA-PALF) reinforced NR prepared with different mastication times were compared. At low level of PALF where aggregation was not a problem, stearic coating had adverse effect on mechanical properties due to the slippery PALF-rubber interface. At high level of PALF, the coating gave composites with higher tensile strength and strain at break. Moreover, tensile strength and strain at break increased with increasing mastication time. This indicates that stearic acid coating reduces the formation of PALF aggregations and allows PALF to work effectively.