An experimental analysis of effects of transversal aspect ratio on air natural convection between horizontal parallel plates, with the lower plate heated, was carried out by means of visualization of flow and air temperature measurement. The distance between the plates, b, in the experiments was 40.0 mm and the width of the plates, W, was 100, 200 and 460 mm. The corresponding transversal aspect ratios, W/b, were 2.5, 5.0 and 11.5. The main flow pattern resembles a C shape (C-loop) in the longitudinal section. In fact, the flow penetrates inside the cavity close to the leading edge of the lower plate and exits from the upper part, by reversing its motion between the plates. The flow visualization and the temperature time records indicate that the lateral walls have significant impact on the secondary flows. As a matter of fact, when the transversal aspect ratio decreases the temperature fluctuations at any location became more regular. The average temperature values, instead do not present marked dependence on W/b.