Copolymerization of phenylene sulfide and phenylene sulfide ether was carried out with various composition ratios. Their thermal properties were investigated by differential scanning calorimetry and thermogravimetric analysis. As the phenylene sulfide ether composition increased in copolymer, glass transition temperature, cold crystallization temperature, melting temperature and melt crystallization temperature all decreased. These results might be due to the flexibility of oxide group of phenylene sulfide ether. When the compositions of phenylene sulfide ether were between 4 and 20% in the copolymer, it showed maximum heat of fusion and heat of solidification. All of the copolymers exhibited excellent thermal stabilities.