Rice is one the newly introduced crop in western Ethiopia (Assosa). However, the yield is far below its genetic potential. Lack of proper management practice on seed rates and row spacing are one of the problems limiting. Field experiment was conducted to investigate the effect of seed rate and row spacing on yield and its traits of lowland rice The treatment consisted of seven levels of seed rate (40, 60, 80, 100, 120, 140, and 160 kg ha<sup>-1</sup>) and four levels of row spacing (15, 20, 25, 30cm). The experiment was laid out as a Randomized Complete Block Design in a factorial arrangement and replicated three times. From the analysis of variance, all the parameters except filled grain were not significantly affected by seed rate and row spacing. The maximum plant height (90.38cm) was observed at seed rate of 40 kg ha<sup>-1</sup> and the lowest was observed at seed rate of 160 kg ha<sup>-1</sup>. The highest panicle length (16.77cm), tiller per plant (3.37) and filled grain (86.93) was observed at seed rate of 40 kg ha<sup>-1</sup>. Filled grain was highly significant and the highest filled grain was observed at 40 kg ha<sup>-1</sup> (86.93) and at 15cm spacing (70.07). The highest grain yield (3.93 t ha<sup>-1</sup>) was obtained at a seed rate of 60 kg ha<sup>-1</sup> and 20cm row spacing. The economic analysis indicated that a seed rate of 60 kg ha<sup>-1</sup> and row spacing of 20cm is the most profitable treatment with mean net benefit of 86,110 Birrha<sup>-1</sup>. Therefore it can be concluded that a seed rate of 60 kg/ha and row spacing of 20cm is preferable and recommended for rain fed lowland rice production areas in the study areas.