The phase diagram of poly(4-vinylphenol)-N,N-dimethyloctadecylamine (PVPh-NDOA) mixture is investigated by X-ray diffraction measurements and optical microscopy. In the low-temperature region PVPh-NDOA mixtures take the mesoscopically ordered phase like a comb-shaped polymer above the melting temperature of the crystal of PVPh-NDOA aggregates. With elevating temperature PVPh-NDOA mixtures undergo an order-disorder phase transition that is a characteristic phase transition in block- or comb-shaped polymers and take homogeneous single phase. Furthermore, at higher temperatures PVPh-NDOA mixtures show the LCST-type phase transition and macroscopically separate into two homogeneous phases. These complex phase behavior in the PVPh-NDOA mixture is explained in relation to the balances of association between the hydroxyphenyl group of PVPh and the dimethylamino group of NDOA, repulsion between PVPh and the octadecyl group of NDOA, and the variation of the entropy contribution with varying temperature.