Experimental and theoretical studies have been conducted on the measurement of the contact potential difference between a powder bed and a metal. A new model was proposed to analyze the experimental data. The model analysis indicated that accurate contact potential difference is obtained only when the electric charge of the powder bed is assumed to be zero. It was found that the absolute value of the contact potential difference decreased with the increase in relative humidity. The contact potential changed with the particle surface modification through methanol vapor. However, the original contact potential difference was obtained again when the methanol evaporated completely from the particle surface.