Thermosensitive poly(organophosphazenes) bearing alpha-amino-omega-methyl-poly(ethylene glycol) (AMPEG), some hydrophobic amino acid esters, and a depsipeptide ethyl ester (ethyl-2-(O-glycyl)glycolate) as a hydrolysis-sensitive moiety have been synthesized. Most of the poly(organophosphazenes) synthesized showed sol-gel transition properties in an aqueous solution. The gelation properties of the polymers were dependent on several factors like the composition of substituents, the chain length of AMPEG, and kinds of amino acid esters. The gelation of the polymers seemed to be imparted by association of hydrophobic groups of amino acid esters. The polymers decomposed under the physiological conditions. The polymer with a small amount of depsipeptide was hydrolyzed more rapidly than that with no depsipeptide. The viscosity of the polymer gels was also affected by their degradation rate.