Self-assembly of PEO-PPO-PEO and PPO-PEO-PPO triblock copolymers at different concentrations were studied in aqueous solution at a fixed temperature of 37℃ while varying the copolymers concentrations. These triblock copolymers self-assembled to form micelles (with sizes <100 nm) and aggregates (with sizes >100 nm). High molecular weight triblock (Mn >5000 g/mol) readily formed micelles above a critical micellization concentration while low molecular weight triblock copolymers (Mn <5000 g/mol) and reverse block sequence copolymers formed aggregates above critical aggregation concentration.