There is great potential for wood plastic composites usage for outdoor applications in the markets. However these products contain natural fibers which go through degradation due to various environmental factors, and that also increase the photo-degradation of polymer matrix. Loss in mechanical properties is one of the undesirable effects of these degradation which by itself can limit structural application of WPCs. Lignin plays an important role in the field of wood weathering. In this study, lignin was partially removed from non-wooden fibers of sugar cane (bagasse). These fibers were applied with high density polyethylene to produce natural fiber composites. Granule polyethylene based pigments were also added to half of the samples. To investigate the effects of delignification and pigments on mechanical properties of produced composites before and after 1440h accelerated weathering, three different mechanical tests were conducted: 3 points bending, vibration test and indentation. These tests were carried out on the samples to obtain the best index for measuring the effect of weathering on samples, and also assess to the changes in MOE, work of maximum load and ductility after exposure to weathering. As well, ATR-FTIR spectroscopy has been applied to explain the chemical changes according to the mechanical ones. An undeniable loss in MOE was observed as the result of using delignified fibers in WPCs. Before any exposure to weathering, interaction of delignified fibres and pigments showed high reduction in MOE. After weathering, the MOE systematically decreased. However, for delignified samples containing pigments minimal change in the MOE was observed. By means of FTIR analysis, delignified samples showed increase in oxidation of surface and produced high amount of carbonyl group after weathering. Also, colored samples showed less decrease in lignin and carbonyl indexes, while they lost more hydroxyl and wood indexes compared to the control samples. These results indicate the role of pigments as reducer of oxidation. Cooperation of delignified fibres with pigments show interesting increase in crystallization of the product after weathering.