The biodegradability of lignocellulosic biomass in anaerobic digesters is the key factor that limits the biogas production. Therefore, scientists are finding the possible ways to reduce the recalcitrant lignin content of biomass that greatly hinder the digestion process. The Photo-oxidation considered advanced oxidation of organic polymers in the presence of optically active catalyst preferably metal oxide. Hence, the present study was designed to report the effects of NiOx along with visible light (350-780 nm) to oxidize the highly lignified wheat straw (WS). Specifically, the experiment was designed to pretreat WS with four different NiOx concentrations under three different exposure times. Consequently, the differentially pretreated samples were subjected to biochemical methane potentials (BMPs) and continuously stirred tank reactors (CSTRs). Further, the pretreatment with 3 wt% NiOx at 4 h light exposure showed 40% (p < 0.05) increase in methane yield during BMP assays and 30% in CSTRs. In addition, the products of lignin oxidation were also quantified as vanillic acid and benzoic were found to be markedly higher (2.6 +/- 0.0 mg/gVS and 0.92 +/- 0.06 mg/gVS respectively) in the maximum pretreated sample that produced. Moreover, the pretreatment with visible light using NiOx nanoparticles was confirmed and it was concluded that light or NiOx alone posed no significant pretreatment of WS. Moreover, the products of lignin oxidation did not halt the efficacy of BMP assays as well as CSTRs. For economical balance the substrate breakpoint was estimated to be 4.53 g per treatment for positive efficacy of the process. (C) 2020 Elsevier Ltd. All rights reserved.