A method for forming three dimensional objects is disclosed. The method includes printing building material, layer by layer, to form a three dimensional object on a printing tray by moving a printing block that comprises a printing head , at least two curing sources and a leveler in forward and reverse passages. The printing is done by moving the printing block in a forward passage when the distance between the printing block and the printing tray is such that the leveler does not touch a layer being built, adjusting a height position of the printing tray relative to the printing block prior to printing a reverse passage and moving the printing block in the reverse passage, wherein during the reserve passage the leveler touches and levels the layer being built.