Novel thermosensitive poly(organophosphazenes) bearing methoxy-poly(ethylene glycol) (MPEG) and amino acid eaters as substituents have been synthesized, and their lower critical solution temperature (LCST) was investigated. Differential scanning calorimetry (DSC) has shown that some of the polymers exhibit crystallinity, which is probably induced by the MPEG side chain of the polymers. Most of the polymers show their LCSTs in the range of 25.0-98.5 degrees C, depending on several factors such as mole ratio of the substituents, molecular weight of the MPEG, and kinds of amino acids and eaters. The more hydrophilic composition of the polymers offers the higher LCST. The LCST of the polymers exhibits almost concentration-independent behavior in the range of 3-30 wt % of the polymers in aqueous solution.