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The sazonal and diurnal patterns of photosynthesis are directly related to biomass production. The objective of this work was to evaluate the variation of photosynthesis in four sugarcane varieties SP79-1011, IACSP94-2094, IACPS94-2101 and IACSP95-5000 growing under field conditions. Measurements of photosynthesis (PN), biomass accumulation and leaf area index (LAI) were taken every month from September 2010 to February 2011, sampling these important physiological traits during the spring and summer seasons. PN was measured on the first fully expanded leaf with visible ligule (+1), every two hours between 7:00 and 17:00 h, considering the natural variation of light availability. The highest values of PN occurred between 9:00 and 13:00 h. Maximum values of PN were around 35 mol CO2 m-2 s-1, regardless sugarcane varieties. Estimates of CO2 uptake throughout spring and summer seasons revealed similarity among SP79-1011, IACSP-95-5000 and IACSP94-2101, with IACSP94-2094 showing lower (~28%) photosynthesis. However, the highest biomass accumulations were observed in IACSP95-5000, IACSP94-2101 and IACSP94-2094 (>250 ton ha-1), with the lowest biomass production occurring in SP79-1011 (~174 ton ha-1). There was not a direct correlation between CO2 uptake per leaf unit area and biomass accumulation. However, we found significant correlation between LAI and biomass production, with LAI ranging between 10.5 and 14.5 in IACSP95-5000, IACSP94-2101 and IACSP94-2094. The lowest LAI value was found in SP79-1011 (7.0). We conclude that biomass production in those sugarcane varieties is explained by canopy development and soil covering rather than to photosynthesis per unit area.