We investigated the solution properties of an amphiphilic poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (PEO-PPO-PEO) block copolymer (Pluronic P105: EO37PO58EO37) in mixed solvents consisting of water and formamide (a nonaqueous polar solvent). The critical micellization concentration and temperature, the thermodynamic parameters of micellization, and the micelle structural parameters were obtained from small-angle neutron scattering (SANS) as a function of the formamide-water ratio, solution temperature, and block copolymer concentration. PEO-PPO-PEO block copolymers self-assemble in formamide-water mixed solvents with increasing temperature, indicating an endothermic micellization. Upon an increase of the formamide-water ratio, the enthalpy and entropy of micellization become smaller than the respective quantities in water. The micelle core and corona radii, the hard-sphere interaction distance of the micelles, the micelle association number, as well as the polymer volume fraction in the core and corona were obtained by fitting a core-corona form factor and a hard-sphere interaction structure factor to the SANS scattering patterns. The micelle radii and association numbers decrease with increasing formamide-water ratio in the mixed solvents. The polymer volume fractions in both the micelle core and corona also decrease with increasing formamide-water ratio. These indicate that the addition of formamide into water affects the micellization process by increasing the solvation of the micelle core and corona, thereby favoring smaller micelles. An increase of temperature results in larger micelle association numbers and a lower degree of solvation in the micelle core and corona in the formamide-water mixed solvents.