By means of direct numerical simulations (DNS) we investigate the effect of a tilt angle β, 0 β π/2, of a Rayleigh-Bénard convection (RBC) cell of aspect ratio 1, on the Nusselt number Nu and Reynolds number Re. The considered Rayleigh numbers Ra range from 10 6 to 10 8 , the Prandtl numbers range from 0.1 to 100 and the total number of the studied cases is 108. We show that the Nu (β)/Nu(0) dependence is not universal and is strongly influenced by a combination of Ra and Pr. Thus, with a small inclination β of the RBC cell, the Nusselt number can decrease or increase, compared to that in the RBC case, for large and small Pr, respectively. A slight cell tilt may not only stabilize the plane of the large-scale circulation (LSC) but can also enforce an LSC for cases when the preferred state in the perfect RBC case is not an LSC but a more complicated multiple-roll state. Close to β = π/2, Nu and Re decrease with increasing β in all considered cases. Generally, the Nu(β)/Nu(0) dependence is a complicated, non-monotonic function of β.