The aim of this study is to investigate the phytochemical constituents of the microalga Chlorella vulgaris using two different cultivation conditions and validate their antioxidant, antiviral, antihyperlipidemic as well as cytotoxic effects against different cell lines; prostate cancer cell line (PC3), hepatocellular carcinoma (HePG2), caucasian breast adenocarcinoma (MCF7) and normal skin fibroblast (BJ1). Alga was heterotrophically grown with a full nitrogen content of the growth medium in both normal and stress growth conditions. Stress was performed by potassium starvation, salting out and ferric chloride in the presence of sodium acetate. Crude protein content for vegetative and stressed C. vulgaris resulted 46% and 30%, respectively. The total carbohydrates of vegetative and stressed types were 25% and 18%, respectively, while the amount of the isolated polysaccharides was 22.6 and 16.5%. Rhamnose (21.27 and 14.89%) and galactose (17.63 and 15.59%) were found to be the main sugars of vegetative and stress alga, respectively; while ribose was the minor identified sugar (3.23 and 1.80%). The total identified fatty acids (7 fatty acids) were 11.25 and 10.29%. Omega 3 (α-linolenic, eicosapentaenoic, and docosahexaenoic acids) and omega 6 fatty acids (arachidonic and linoleic acids) were present in vegetative and stressed alga. Fat- and water-soluble vitamins showed that vitamin E is the major one (181.24 and 49.17 mg.100g-1) following by vitamin B complex (48.34 and 35.05 mg.100g-1). In addition, 6 main pigments and 11 phytosterols were also detected. Total chlorophyll decreased in the stressed C. vulgaris, while content of total carotenoids showed an inverse trend comparing to the vegetative one. Also, the stressed pigment fraction of C. vulgaris exhibited the strongest antioxidant activity against both DPPH and ABTS (32.30 ± 0.51 and 44.44 ± 0.84mg/g), respectively. Contradictory, the vegetative polysaccharides fraction exhibited anti-proliferative effect against prostate cancer cell line and hypolipidemic effect on β-hydroxy-β-methylglutaryl Coenzyme A reductase enzyme; the key enzyme of cholesterol biosynthesis. The vegetative pigment fraction showed the highest inhibition effect on the propagation of influenza virus by 96.10%. In conclusion, the stressed pigments fraction of C. vulgaris recorded in vitro antioxidant effects, while vegetative polysaccharides fraction showed hypolipidemic, anti-proliferative and anti-prostate cancer. Additionally, the vegetative pigments fraction recorded anti-influenza virus effect.