A series of poly(N,N-diethylacrylamide) samples with low molecular weights (1.9x10(3)-5.3x10(4)) and narrow polydispersities (below 1.5 and usually lower than 1.25) was synthesized by reversible addition-fragmentation chain transfer polymerization. The phase separation behavior of poly(N,N-diethylacrylamide) in aqueous solution was investigated by turbidimetry, fluorescent probe technology and DSC. It is interesting to find that the lower critical solution temperature (LCST) of the samples increases with increasing molecular weight and remains more or less a constant above a critical molecular weight of 1.2x10(4). At the same time, an inverse dependence of LCST on the concentration was found and this effect was more pronounced for lower molecular weight. To further investigate the novel molecular weight dependence of the LCST, the fluorescent probe study was conducted and the experimental results demonstrated that there was an increase in hydrophobicity when decreasing the molecular weight and increasing the concentration and flower-like micelles were probably formed which can further be proved by TEM.