This study investigates the effect of rotating horizontal single or multi cylinders on mixed convection heat transfer in an equilateral triangular enclosure filled with air. The governing equations for the steady, laminar, two dimensional, incompressible flows with Boussinseq approximation and constant fluid properties are solved numerically using the finite element method with FlexPDE soft package. Three cases are performed: single rotating cylinder, three rotating cylinders at the same direction and three rotating cylinders at different directions. The main parameters are: Rayleigh number ( ), Prandtl number ( ), the dimensionless angular velocity (Ω=0-1000) (for both directions clockwise CW and counter clockwise CCW) and dimensionless radius of rotating cylinder (R=0.1-0.25). It was found that the average Nusselt number for the single or multi rotating cylinder is increased with increasing Ra, R and Ω for all cases. Also the average Nusselt number of single rotating cylinder is greater than the multi rotating cylinders for the same ratio of the solid cylinder or cylinders volume to total enclosure volume. The results are compared with other authors in the literature and a good agreement was seen.