The flax and equivalent proportion of poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) were melt compounded and injection molded to prepare flax-reinforced polylactide stereocomplex (sc-PLA) bio-composite, and the effect of alkali treatment on the structure and properties of flax as well as the flax/sc-PLA composite was investigated. SEM and FTIR results showed hemicellulose in flax was almost completely removed after alkali treatment and the treated flax (ALK-flax) bundles were more separated with a cleaner surface than untreated flax (UN-flax). DSC results showed homo-crystallites (hc, T-m=160-170 degrees C) and stereocomplex crystallites (sc, T-m approximate to 210 degrees C) coexisted in sc-PLA and flax/sc-PLA composites. Compared with sc-PLA, the total crystallinity and sc-crystallinity of flax/sc-PLA composite increased regardless of whether the flax were treated with alkali, whereas ALK-flax/sc-PLA composite showed a little higher crystallinity than UN-flax/sc-PLA composite. TGA results confirmed ALK-flax/sc-PLA composite had a higher thermal degradation temperature than UN-flax/sc-PLA composite. The mechanical tests indicated although the mechanical properties of sc-PLA increased significantly by reinforcing with flax, the ALK-flax/sc-PLA composite showed little lower mechanical properties than UN-flax/sc-PLA composite. The alkali treatment of flax had no obvious influence on the Vicat softening temperature (VST) of flax/sc-PLA composites, a higher heat resistance with VST at approximate to 155 degrees C could be obtained for flax/sc-PLA composite. POLYM. ENG. SCI., 55:2553-2558, 2015. (c) 2015 Society of Plastics Engineers