The purpose of this study was to examine the relationships between running performance and the cross–sectional area of the psoas major, peak oxygen uptake, and running economy in male junior long–distance runners. The subjects were 37 male junior athletes who achieved good records in interscholastic athletic competition during 5 years (2011–2015). Their seasonal best times in a 5,000 m race (5,000m–SB) were 14:04.11 ± 0:07.25 (13:53.64–14:16.15). In a multiple regression analysis, 5,000m–SB was statistical significantly correlated with the cross–sectional area of psoas major (16.0 ± 1.7 cm²) measured on magnetic resonance imaging and peak oxygen uptake (4.25 ± 0.36 l min^-1 [76.9 ± 5.8 ml min^-1 kg^-1]) during a lactate curve test comprising five stages. However, 5,000m–SB was not related to oxygen uptake at the work–load which was less than the load at the lactate threshold estimated by using the lactate curve test results. These results suggest that a high volume of the psoas major, the largest hip–flexor muscle, and peak oxygen uptake are among the important factors for junior long–distance runner performance.