The present study was carried out to study the long term effect of different nitrogen forms and concentrations (14.3, 28.6 and 57.1 µM N-NH4 and 214, 2360 and 3700 µM N-NO3) on the photosynthesis and relative growth rate (RGR) of Ulva sp. for five weeks. Nutrient-enriched seawater was supplied at an exchange rate of between 12.6 and 14.4 volumes per day. Relative growth rate was determined weekly. Photosynthetic oxygen evolution response was measured using 20 - 25 mg fresh weight seaweed incubated in a DW3 measuring chamber for 56 min under different irradiances with light and dark periods alternating every seven minutes. Photosynthetic light-response curves were drawn using 15 photosynthetic oxygen evolution readings normalized to dry weight. Experimental data were fitted with a Haldane model to calculate photosynthetic rate (Pmax), saturation irradiance (Is) and compensation irradiance (Ic). At the experiment termination, the RGR following nitrate addition were higher, but not significantly, than those with the ammonium supplied. The highest RGR was determined at the medium nitrate concentration. The photosynthetic activity of Ulva sp. showed a positive relationship with nitrogen concentrations from both nitrogen forms. The highest significant Pmax was found at the highest nitrate concentration. The lowest ammonium concentration corresponded to the lowest significant Pmax value. No significant differences were found for Is and Ic irrespective of treatment, although, in general, ammonium treatments yielded higher Is values than nitrate treatments. The discrepancy between the growth rate results and photosynthetic oxygen evolution is discussed in light of the reproduction activity and temperature effect.