This study was conducted to evaluate the effect of different bottom substrates on growth rate and quality of sea grape (Caulerpa lentillifera) cultivated in tanks. Five different bottom substrates, namely hard (plastic tank bottom), sandy, muddy, sandy-mud, and muddy-sand substrate were applied in the 500-L plastic rectangular tank. Each tank was divided into 3 equal compartments considered as 3 replications that was set up under semi-transparent roof at salinity of 30 ppt and provided slightly continuous aeration. Initial stocking rate was 15 g/compartment and fishmeal was used as nutrient source for C. lentilifera. After 36 days of cultivation, daily temperature and pH in experimental tanks fluctuated in the ranges of 26.2-30.5oC and 7.9-8.7, respectively. Growth rate of C. lentillifera cultivated on hard bottom and sandy substrates (3.14%/day; 3.12%/day, respectively) were significantly higher than in other substrates. The proportion of erect branch (edible part)/thallus was from 56.0 to 65.6% in which the lowest value was observed in hard substrate. Regarding quality of C. lentillifera, the percentages of the erect branch length from 5 cm onwards (commercial size) was lowest in the hard substrate treatment (41.8%) and highest in sandy-mud substrate (70.7%), other substrates had intermediate values that was not significantly different (p>0.05) from the sandy-mud bottom. Moreover, visual observation found that C. lentillifera cultivated in the sand substrate had the erect branch with more densely ramulus and brighter green in color as compared to those cultivated in other substrates. These results indicated that sandy substrate can be suitable for cultivating Caulerpa lentillifera in tank.