Abstract: When H. pluvialis was inoculated in a nitrogen-deficient medium and exposed to high light intensity condition(10—12 klx), the change of green motile cells to red motile cells was observed in four days. After four days of inoculation, the number of vegetative cells was 90% of total number of cells and the astaxanthin content(33.0 pg/cell)was 55% of the final astaxanthin content(60.0 pg/cell)which was inoculated ten days. After six days, the astaxanthin content was 75% of final astaxanthin content. It was found by electron microscopic observation that the cytoplasm of red motile cells was almost full of pigment granules, but the morphology and structure of cell was identical with green motile cells. The experiments also showed that the astaxanthin accumulation in motile cells would occur too when H. pluvialis was exposured to nitrogen starvation but lower light intensity condition(0.5—1.0 klx). When H. pluvialis was only exposured to high light-stress condition(inoculated in complete medium), the large amount pigment accumulation was only found during the transformation of vegetative cells to akintes.