Abstract: In order to determine the best ecological conditions for the growth of Gracilaria blodgettii, we observed the changes in its relative growth rate (RGR) and biochemical constituents at different total inorganic nitrogen concentrations (48 μmol·L^-1, 96 μmol·L^-1, 144 μmol·L^-1, 192 μmol·L^-1, 240 μmol·L^-1, 288 μmol·L^-1, 336 μmol·L^-1) and with different N/P values (1/1, 5/1, 10/1, 50/1, 100/1). The optimum total inorganic nitrogen concentration was 192 μmol·L^-1; the optimum N/P value was 10/1;the optimum m(NO₃^--N) : m(NH₄⁺-N) : m(NO₂^--N) were 1 : 10 : 5 and 5 : 10 : 1. Under these conditions, G.blodgettii obtained higher RGR, the highest contents of phycobiliprotein and soluble protein, but lower peroxidas(POD) activity and malondialdehyde (MDA) content. However, the algae suffered malnutrition and slow growth when total inorganic nitrogen concentration was too low, N/P value was too high and the ratio of NH₄⁺-N to total inorganic nitrogen was relatively low.