To evaluate the effects of herbicide rate, cultivar, and spatial pattern on rice grain yield and weed suppression, a field experiment was conducted on a lowland rice field at the Rice Research Station of Tonekabon, north of Iran, in 2011. The experimental design was a split plot-factorial where the whole plot portion was a randomized complete block with three replicates. Main plots were pretilachlor rates (0, 0.5, 1, 1.5, 2 L ha -1 of pretilachlor). The subplots were factorial combinations of two crop spatial patterns (uniform and wide-row planting patterns, 20× 20 and 30 × 13 cm, respectively) and two traditional rice cultivars ('Hashemi' and 'Deylamani'). Regardless of cultivar and spatial pattern, rice grain and biological yields, panicle number per m 2 , leaf area index, and herbicide efficacy increased, while weed biomass decreased with increasing pretilachlor application rate. Rice grain and biological yields, panicle number per m 2 , leaf area index, and herbicide efficacy were significantly greater in uniform planting pattern than in wide-row planting pattern when averaged over cultivars and pretilachlor rates. Moreover, uniform planting pattern of rice plants suppressed weeds 39% better than wide-row planting pattern as averaged across pretilchlor rates and cultivars. 'Deylamani' produced grater grain and biological yields, panicle number per m 2 , leaf area index than 'Hashemi' as averaged across pretilachlor rates and spatial patterns. This experiment illustrated that uniform spatial pattern increased weed suppression, herbicide efficacy, and grain yield.