Thermosensitive poly(organophosphazenes) bearing α-amino-ω-methoxy-PEG (AMPEG) and hydrophobic L-isoleucine ethyl ester (IleOEt) as side groups have been synthesized, and their reversible sol-gel properties were investigated by means of 31P NMR spectroscopy and viscometer. In an aqueous solution, the poly(organophosphazenes) exhibited four-phase transitions with temperature gradually increasing: a transparent sol, a transparent gel, a opaque gel, and a turbid sol. The gelation properties of the polymer were affected by several factors such as the composition of substituents, the chain length of AMPEG, and the concentration of the polymer solutions. The more hydrophilic composition of the polymers offers the higher gelation temperature. The gelation of the polymer is presumed to be attributed to the hydrophobic interaction between the side-chain fragments (-CH(CH 3)CH 2CH 3) of IleOEt which act as the physical junction in the polymer aqueous solution.