Effects of part replacement (0, 15, 30, 45, 60 g/100 g) of rice flour by buckwheat flour (BF) on gel structure, cooking characteristics, nutritional compositions, and in vitro starch digestibility of extruded rice-buckwheat noodles were investigated. The visocity profile of blended flour showed that peak viscosity and setback value significantly increased with increasing substitution levels of BF. Scanning electron microscope images indicated that 30 g/100 g BF replacement induced the formation of more integrated starch gel network. The cooking loss and broken rate of extruded noodles first decreased and then increased with the incorporation of BF. The lowest cooking loss and broken rate were observed at 30 g/100 g BF replacement. BF substitution increased the content of polyphenols and flavonoids in extruded noodles and higher retention rate of polyphenols and flavonoids was found after extrusion cooking when substitution level of BF was at 30 g/100 g. Finally, the predicted glycemic index (pGI), and the amount of rapidly digested starch both decreased, while the amount of resistant starch increased with increasing BF proportions, indicating BF is a suitable ingredient to make low GI products. •BF enrichment greatly affected gel microstructure and quality of rice noodles.•A more integrated starch gel network was obtained at 30 g/100 g BF replacement.•The lowest cooking loss and broken rate were observed at 30 g/100 g BF replacement.•The highest retention rate of TFC and TPC was observed at 30 g/100 g BF level.•BF addition increased content of dietary fiber and decreased pGI values of noodles.