This study was conducted in lowland fields in a cool temperate region of Japan to determine a simple method to evaluate the suppressive ability of rice (Oryza sativa L.) cultivars against Echinochloa oryzicola Vasing. The optimal timing of E. oryzicola (0.5 leaf stage) mixed planting to detect the difference between the weed suppressive ability of different rice cultivars was at 10–17 days after rice transplantation. Rice traits measured at early growth stage did correlate better to E. oryzicola biomass at harvest than traits measured at late growth stages. The relationship between rice traits and E. oryzicola biomass was consistent across two weed competition regimes (presence or absence of E. oryzicola). Thus, rice cultivars’ traits measured in weed-free plots were adequate to evaluate their weed suppressive ability. An evaluation experiment, using the 26 rice cultivars in monoculture and mixed culture with E. oryzicola, was conducted. Rice traits and light interception were measured between 26 and 55 days after rice transplantation. The relative photosynthetic photon flux density (R-PPFD), rice biomass and the plant volume (plant height × longest diameter × shortest diameter of rice canopy) measured at 40 days after rice transplantation in monoculture plots were strongly correlated with E. oryzicola biomass at harvest. R-PPFD and these biometric traits were good predictors of the suppressive ability of rice cultivars against E. oryzicola in a cool temperate region of Japan. In view of simplicity, the plant volume of rice was the most useful trait for the evaluation.