Real-world aerosols often vary over time. A time-varied aerosol generation system is thus needed to evaluate the performance of aerosol sensors for characterizing time-dependent aerosol size distribution. However, all laboratory aerosol generators are designed for the production of aerosols with stable concentrations and steady size distributions. A simple way to produce reliable time-dependent aerosols is to combine a stable aerosol generator with a programmable aerosol diluter. Aerosol diluters are also needed to measure aerosols in high concentrations using existing aerosol instruments. In this study we focused on the design and evaluation of a programmable aerosol diluter. The dilution flow rate of the aerosol diluter was controlled by a programmable mass flow controller with a flow rate of up to 200 L/min. Steady and dynamic dilution processes in the diluter were programmed using Visual Basic. Experiments were carried out to characterize the steady and dynamic dilution performance of the aerosol diluter for DMA-classified particles with sizes ranging from 10 nm to 1.0 µm. The steady dilution result shows that the diluter has a non-size dependent dilution performance, and there is a good linear relationship between the aerosol dilution ratio and dilution flow rate ratio (with a calibrated line slope of 1.03, close to the ideal line slope of 1.0). Our experiments further indicate that efficient aerosol mixing in the diluter can be achieved when operated at a flow Reynolds number above 450. The evaluation of four dynamic dilution modes also provides evidence of the excellent performance of the diluter, which has the capability of continuously producing well-defined and concentration-varied aerosols. A simple empirical model was also proposed to describe the steady and dynamic dilution performance of the diluter.