The present invention relates to the soluble glucan oligomer having a M.W. ranging from 1,000 to 10,000 prepared by treating insoluble beta-glucan isolated the cell wall of yeast variant IS2 with commercially available beta-glucan hydrolyzing enzymes. The soluble glucan oligomer showed potent efficacy on promoting immune activity and on inhibiting the growth of cancer cell, therefore, it can be used as the therapeutics or health care food for treating and preventing immunodeficiency and cancer disease.