The aspect ratio effect on natural convection heat transfer in a rectangular enclosure with protruding heat sources has been experimentally investigated. Five protruding heaters were mounted with uniform vertical spacing on one vertical wall. The vertical wall opposite to the wall on which heated sections were mounted was movable so that the enclosure width could be adjusted to the desired value. The top surface of the test enclosure was an isothermal heat sink. All other surfaces except the two end vertical surfaces were insulated. The five heaters were identical with each having horizontal protuberance of L3 = 9 mm and vertical height of L1 = 15 mm. The vertical spacing between the heaters was L2 = 15 mm. The enclosure width was varied in experiments from W = 13.5 mm to 45 mm. The experiments were conducted for six values of cavity width resulting in variations in the cavity height-to-width ratios (aspect ratios) and cavity width-to-protruding heater height ratios of 3.67 to 12.22 and 1.5 to 5.0, respectively. Ethylene glycol was used as the convective medium. Flow visualization pictures and heat transfer data indicate that the starting point of core flow directly affects the local heat transfer coefficient of the bottom heater, while the secondary flow cell between the top heated section and the top sink surface influences the heat transfer coefficient of the top heater. Cavity width variation influences the heat transfer process mainly through altered flow patterns. This influence is weak when the ratio of width-to-protruding height is 4.0 and negligible when this ratio is 5.0 or more. Based on the local height length scale (measured from the bottom of the cavity) the data for all the cavity widths are correlated and an explicit relation for the aspect ratio effect on local Nusselt number is reported. The correlation of local Nusselt number versus local modified Rayleigh number is independent of the number of heaters in the vertical array, cavity width-to-heater protrusion height ratio, W/L3, and vertical height location of the heaters. This conclusion is based on the present results and previously reported data and is valid for the following conditions: 1.5 ≤ W/L3 ≤ 5.0; 3.67 ≤ aspect ratio ≤ 1.22; vertical height of heater from 8 to 15 mm; and number of heated sections from 5 to 10.