The level of relative humidity is one of the key parameters in evaluating indoor air quality and comfort. In principle, humidity can be kept more uniform over time by use of materials that adsorb moisture from the air reversibly. This study was conducted to investigate the effect of loess treatment and carbonization on the hygric performance of medium-density fiberboard (MDF). The loess treatment was conducted with different sizes of loess particle prepared by a high-pressure homogenizer. After loess treatment on the surface of the MDF, it was carbonized at high temperature (600 °C). Loess is an abundant mineral high in Si content, which has high moisture absorption capacity, which remained after the carbonization process. The study also found that the loess treatment positively affected the hygric performance of carbonized MDF (c-MDF). The hygric performance of c-MDF almost doubled after the loess treatment compared with the non-treated c-MDF. However, the nano conversion of loess did not influence the hygric performance. Loess-treated carbonized MDF could be used as a humidity controller in buildings.