In order to analyze the effect of the growth direction on dendrite arm spacing (1) and microhardness (HV) during horizontal directional solidification (HDS), experiments were carried out with the Al-3wt.%Cu-5.5wt.%Si alloy and the results compared with others from the literature elaborated for upward directional solidification (UDS). For this purpose, a water-cooled directional solidification experimental device was developed, and the alloy investigated was solidified under unsteady-state heat flow conditions. Thermal parameters such as growth rate (VL) and cooling rate (TR) were determined experimentally and correlations among VL, TR, 1 and HV has been performed. It is observed that experimental power laws characterize 1 with a function of VL and TR given by: 1=constant(VL) -1.1 and 1=constant(TR) -0.55 . The horizontal solidification direction has not affected the power growth law of 1 found for the upward solidification. However, higher values of 1 have been observed when the solidification is developed in the horizontal direction. The interrelation of HV as function of VL, TR and 1 has been represented by power and Hall-Petch laws. A comparison with the Al-3wt.%Cu alloy from literature was also performed and the results show the Si element affecting significativaly the HV values.