The effects of environmental factors were investigated in fractal dimension (D) of Gracilaria tenuistpitata. G. tenuistipitata was cultured indoors under various factors for 30 days. Algal thalli were pressed on flat surface for photopraph with optical or digital camera to measure fractal dimension based on the relationship between levels and numbers of algal branch. Environment factors in this study were including illumination (L¡F 45~365£gmol¡Em-2¡Es-1), water flow (F¡F 80~900L¡Eh-1), temperature (T¡F 10~35¢J) and salinity (S¡F 10~50 ppt). The results of ANOVA and General linear models showed that illumination, temperature and salinity but water flow significantly affected thallus fractal dimension with the maximum at 255£gmol¡Em-2¡Es-1, 19¢J and 24 ppt respectively, The cross reactions between environment factors did not significantly affect the fractal dimension, reflecting that environment factors affect the appearance of G. tenuistpitata independently. The effect of environment factors on biomass of G. tenuistpitata was studied in each experiment and the results showed that all environment factors tested in this study significantly affected the biomass of G. tenuistpitata.