The Little theater in Sejong Performing Arts Center is a proscenium theater with 447 seats, which was built in 1978. Acoustical remodeling design of the theater was carried out to improve the acoustic quality in audience area and to meet functional requirements of the theater. Design strategies included use of more reflective surfaces, installation of diffusers on sidewalls and orchestra pit, and side balconies. In particular, sidewall diffusers were designed through measurements of scattering coefficients according to ISO standardAfter remodeling, field measurements were performed to validate the acoustical design. The results was shown that unoccupied reverberation time varied from 0.93 to 1.06 s, and 1-IACC (Interaural Cross-correlation Coefficient) was more than 0.60. It was also found that sidewall diffusers resulted in decreases of COV (coefficient of variation) of early decay time and 1-IACC and increases of and Np (number of peak). Furthermore, the balance, refers to the sound power ratio of the stage and the pit sources, was -0.2 dB.