Natural convection heat transfer from horizontal cylinders of different shapes in a rectangular enclosure has been investigated numerically. In the present study, isothermal horizontal cylinder of circular, square and triangular shapes having same heat transfer surface area have been simulated in laminar regime for Rayleigh number in the range of (10³ ≤ Ra ≤ 10⁶). The present numerical analysis is able to predict the natural convection flow pattern and thermal field around different horizontal cylinders in terms of velocity vectors and temperature contours. Average surface Nusselt number has been calculated and validated with the experimental and numerical results of G. Cesini et al. [1]. For the same Rayleigh number, variation of average surface Nusselt number and heat flux for different cylinders have also been presented. It has been observed that for a particular Ra, average surface Nusselt number is higher for circular cylinder compared to triangular cylinder followed by square cylinder. A correlation has also been developed for average Nusselt number for all the cases of cylinders.