Objective: The effect of polymers used in nanoparticle (NP) production on the formulation properties is one of the few studied issues. Therefore, this study aims to formulate flurbiprofen (FLB) loaded NPs with different molecular weight (M-w) poly lactic-co-glycolic acid (PLGA) and investigate the effect of M-w on NP character. One of the most important objectives is to provide a high anti-inflammatory effect with a low dose and the anti-inflammatory efficacy of the selected optimal formulation is to be determined by in vivo hen's egg test on Chorioallantoic Membrane (HET-CAM) analysis that a new, popular and in vivo animal experiment alternative method. Significance: To determine the anti-inflammatory efficacy of the optimum formulation by HET-CAM analysis. To the best of our knowledge, this is the first report on the in vivo anti-inflammatory evaluation of FLB-loaded PLGA NP using the in vivo HET-CAM assay. Methods: Blank and FLB-loaded PLGA NPs were prepared using a nanoprecipitation technique. The cell viability test for all formulation was performed with MTT in the NIH-3T3 mouse embryonic fibroblast cell line. The anti-inflammatory activity of optimum formulation (A(6)) was examined using the in vivo HET-CAM assay. Results: The particle sizes (PSs) of the FLB-loaded PLGA NPs were between 175 and 198 nm. The encapsulation efficiency (EE%) was a range of 82-93%. In vitro release of NPs showed extended-release up to 144 h. The release kinetics were fitted to the Peppas-Sahlin and Weibull models. The results showed that PS, PDI, EE%, and release rates of NPs were directly related to the M-w of PLGA. There is no statistically significant difference in cell viability study was observed between blank and FLB-loaded PLGA NPs. The in vivo anti-inflammatory activity results indicated that A(6) coded formulation was showed significantly good anti-inflammatory potential at low dose. Conclusions: It could be concluded that FLB-loaded NPs seem to be a promising extended-release drug delivery system for oral administration with a low dose and high efficiency.