In this study, a poly(lactic acid)(PLA) and ionomer blend system was investigated to extend the service temperature of PLA to higher temperatures. In the first study, a PLA/sulfonated polystyrene(SPS) ionomer blend was fabricated using a SPS ionomer having a relatively higher Tg than other PS-based ionomers. In addition, the effect of SPS ionomer on the mechanical properties of the PLA blend was investigated. In was found that the PLA/SPS ionomer blend was a phase separated blend. It was seen that PLA acted as a multiplet-plasticizer for the SPS ionomer and the SPS ionomer acted as a filler for PLA, depending on the relative amount of the two materials in the blends. On the other hand, to further secure the shape stability of PLA at high temperature, SPPhO ionomer prepared by sulfonating PPhO, a heat-resistant polymer, was used to make PLA/SPPhO blend. It was found that the SPPhO ionomer acted as a filler for PLA and could be much more helpful in increasing the service temperature range of PLA than the SPS ionomer. Meanwhile, several models related to the blend system were applied to the data to interpret the mechanical properties of PLA/sulfonate ionomer blends; Guth equation, parallel model, series model, logarithmic, Nielsen equation for the regular system and inverted system. In the second study, to improve the miscibility between PLA and ionomer, PSMA ionomer, having relatively weaker electrostatic attraction between ion pairs than SPS ionomer, was used to prepare a blend with PLA. It was observed that the Tg of the PLA/PSMA ionomer blend was about 70 °C higher than that of pure PLA. This suggested that the thermal properties of pure PLA were improved due to the interaction occurring between the two materials. However, with the addition of the PSMA ionomer, the crystallinity of PLA rapidly decreased, and accordingly, the storage modulus of the blend rapidly decreased at about 90 °C. Therefore, it could be propered that PSMA ionomer might help to improve the thermal properties of PLA, but it was not very helpful in improving the mechanical properties of the PLA.