The objective of this study is to investigate the effects of alkali treatment on the mechanical, physical properties of cupula fibers reinforced recycled high density polyethylene (r-HDPE) composites. The composites were produced with different chestnut (Castanea sativa Mill.) cupula fiber contents, up to 50%, and polyethylene-grafted maleic anhydride. The mechanical, physical, thermal, and morphological properties of the treated and untreated samples were investigated. In addition, the Fourier transform infrared spectroscopy (FTIR) was used to observe the interactions and functional groups between cupula fiber and r-HDPE in the composite samples. The alkali treatment with 5 wt% NaOH solution had an affirmative effect on the tensile and flexural properties. It was found that an increase of 20% in flexural strength values with up to 40 wt% alkali treated cupula fiber loading in polymer matrix compared to that of untreated samples. After 5 wt% NaOH treatment, the crystallinity decreased with the increase of the chestnut cupula fiber ratio. As excepted, the FTIR analysis showed that the alkali treatment resulted in removal of hemicellulose, and lignin from cupula fiber. The results revealed that the alkali treated chestnut cupula fiber can be used successfully in polymer composites as a natural fiber filler.