Heat transfer by natural convection in parallelogrammic enclosures was studied both experimentally and theoretically. Two sides of a parallelogram, placed in the direction parallel to the gravity, were chosen as heating and cooling surface. The remaining two sides were tilted either upwardly or downwardly according to the relative position of the two vertical walls. Heat transfer coefficients and flow patterns were obtained under various thermal and geometrical conditions. It was recognized that, for two identical parallelogrammic enclosures, the heat transfer rate in the upward tilt enclosure (of which the high temperature wall was placed at lower position than the low temperature wall) was much larger than that in the downward tilt enclosure. The ratio of the two heat transfer rates was as large as eleven when the angle of tilt was 45 degree. The results of numerical analysis using the Galerkin method agreed well with the results of heat transfer measurement and flow visualization.