This study analysed the effect of linear slot diffuser length changes and proposed an optimal length for proper air supply in an office room. To evaluate the performance, the computational fluid dynamics (CFD) simulation is used for the validation with the measurement data. For investigating the performance of linear slot diffuser as an air supply terminal, the air diffusion performance index (ADPI) and CO2 concentration were selected as performance indicators. Numerical results showed that the longer the length in comparison with the width of a diffuser gets, the more ADPI becomes increasing. It is also found that the linear slot diffuser helps mixing the air in the test room but is not well performing for indoor air quality. Therefore, when a linear slot diffuser is used, it is necessary to discharge CO2 from occupants directly.