This investigation was proceeded to define the effects of gibberellic acid on the growth of Chlorella by determining the contents of chlorphyll and changes in various components in Chlorella cells according to the concentration of treated GA. The growth of Chlorella was accelerated with telative low concentrations of GA(10, 40 ppm) and was restrained with relative high concentrations of GA(70, 100, 200 ppm). The synthetic ability of chlorophyll of GA was inhibited generally in proportion to the concentration of treated-GA and the higher the concentration of GA was applied, the longer time was required in the restoration. The contents of RNA, protein and soluble carbohydrate were increased PCA-soluble amino acid and polysacharide were decreased in those cell components between the accelerated and restrained group. Consequently, the effect on accelerated growth in relative low concentrations of GA is considered to be caused by the powerful effet on expansion growth of GA. It is presumed that the effect of restrained growth in relative high concentrations of GA is due to the inhibitory effect on the chlorophyll synthesis.