The weathering of wood-plastic composites (WPC) causes discoloration which affects their aesthetic aspects. It has been proved that these discolorations are due to lignin degradation. Effects of blocking the susceptible structure of lignin assessed by chemical treatment such as acetylation and methylation are reported in this study. Surface chemical change of wood plastic composite (WPC) formulations based on high density polyethylene (HDPE) were monitored by means of ATR FT-IR spectroscopy and colorimetry. According to the standard ASTM 2565, samples were placed in Atlas Xenon apparatus for 250 hrs and 2000 hrs. The results have shown that methylation and acetylation can photostabilize lignin in short period of times. ATR FT-IR spectra shown that, in long term, none of the treatments could protect lignin degradation, within wood flour. Methylation limited the depth of penetration of weathering and these samples have more lignin content compared to control samples.