The cooling capability in compact electronic equipment of natural convective flow fields was investigated. A relationship between air passage width in the channel and natural cooling capability was obtained. Temperature and velocity measurements were carried out using a channel model of electronic equipment comprising a vertical duct with a rectangular cross-section. The channel model comprised two copper walls and two transparent acrylic walls. The clearance between the copper walls was used as a parameter in the channel model. Velocity profiles of natural cooling flow in the channel were quantitatively measured using particle image velocimetry (PIV). The temperature and velocity results demonstrated that changes in the velocity profiles closely depend on the wall clearances. Our studies indicate that a clearance of 8 to 10 mm is optimal for natural cooling in a channel of this type with the 56 mm channel depth.