The experiments were conducted to determine suitable sowing time in order to achieve high plant biomass and sugar content of sweet sorghum for bioethanol manufacture. The results showed that germination rate reached > 80% in all trial times (p > 0.05). The growth and development speed of sweet sorghum reduced when seeds were sowed in August and was significantly different from other sowing times (p < 0.05). Sowing from March to June obtained the highest plant height during all growth and development stages. Lodging and diseases observed in all periods of sowing, and planting began in July and August had lower percentage. In contrast, the productivity of fresh weight (1310.4 g/whole plant), sugar content (14.9% Brix), biomass yield (122.4 tons/ha) and theoretical converted ethanol yield (5 tons/ha) were the highest when sweet sorghum planted from March to June. It was observed that sowing sweet sorghum in four periods of month from March to June had the desirable biomass for bioethanol production.