Amphiphilic block copolymers of ethylene oxide and caprolactone were synthesized for three compositions of 5 k-2.5 k, 5 k-5 k, and 5 k-7.5 k(PEO-b-PCL) and micellar aggregates were formed in aqueous medium with the particle size in the range of 0.045-0.118 ㎛. The feasibility of the micelle solutions was studied in terms of colloidal stability, biodegradability and in vitro toxicity for intravenous (Ⅳ) drug delivery carriers. It was found that the colloids were stable in neutral PH PBS solution for 180 days at 37 ℃ while large aggregates were formed in acidic medium of pH 1.2-3.5. Enzymatic degradation of the polycaprolactone core was evidenced in the presence of lipase and serum as monitored by carboxylic acid generation, however protease showed little activity on the degradation of the particles. The colloidal stability of the micellar aggregates was maintained at the particle concentration less than 0.025 wt/v% in serum. In the animal toxicity test, 0.7 mL of micelle solution of 5 k-7.5 k block copolymer (1, 2 wt/v% solid) was intravenously administered to 6-wk old male mice and the general symtom was observed. This study demonstrated that most of animal subjects survived showing normal weight gain after receiving six times of single dose by 1 h period.