There is a noticeable gap in knowledge about the key phases for grain yield determination in temperate legume crop species such as lupin and pea. For this reason, the aim of the present study was to comparatively assess the critical period for grain yield and grain number determination of narrow-leafed lupin (Lupinus angustifolius L.) and pea (Pisum sativum L.). Two field experiments were conducted during the 2007–08 (Exp. 1) and 2009–10 (Exp. 2) growing seasons at the experimental station of the Universidad Austral de Chile. Treatments were the combination of (i) two crops (narrow-leafed lupin and pea) and (ii) seven shading treatments (control without shading and six shading treatments imposed for 15 days throughout the crop cycle). Lupin showed a longer (P < 0.01) crop cycle than pea, mainly related to the shorter reproductive period of pea. Above-ground biomass, grain yield and harvest index of lupin and pea were significantly (P < 0.01) affected by shading in both experiments, though the impact of treatments varied with the timing of the source reduction. Considering a bottom line of 10% reduction in grain yield, the critical periods for grain yield determination were established between 10 days before R1 and 50 or 40 days after R1 for lupin and pea, respectively. Owing to the significant (P < 0.01) and strong relationship shown by grain yield and both (i) grain number and (ii) pod number per square meter, these three key crop traits showed similar temporal windows for their determination. Crop differences in the critical periods were minimized when the key windows of lupin and pea were expressed relative to the timing of physiological maturity. Definition of the critical period of grain yield determination is useful for maximizing grain yield of these crops based on breeding or management strategies.