We discuss how hydrophobic polyether sulfone hollow fiber membranes could be hydrophilized by surface coating them with natural hydrogel calcium alginate. The outer surface of the hollow fiber membrane was brought into contact with a solution of sodium alginate while calcium chloride solution was held within the lumen of the membrane. The divalent calcium ions diffused through the membrane pores and cross-linked the alginate, which formed a coating layer on the outer surface. The thickness of the calcium alginate layer could be controlled by changing the duration of cross-linking. The coated hollow fiber membrane samples were characterized by transmission electron microscopy (TEM). The increase in surface hydrophilicity was confirmed by water contact angle measurement. The hydraulic permeability of the coated membrane was higher than that of the uncoated membrane. The permeability of model solutes such as dimethyl sulfoxide (DMSO) and antipyrine was similar with both coated and uncoated membranes. Such hydrophilized hollow fiber membranes could potentially be used in membrane bioreactors and as scaffold for tissue engineering.