The stability of a river-bed protection block in a flood event is affected by underscouring of the block. The effects of block spacing on block stability were investigated in different underscour conditions. Three blocks were placed at the bottom in a flume, and the underscour depth of the frontal block and distance between the first and second blocks were changed. Drag and lift forces were measured directly in each experimental condition. Particle Image Velocimetry (PIV) was used to visualize the flow and eva- luate the streamline pattern. Pressure measurements around the block to understand the pressure distribution show that the pressure distribution on the rear and bottom surfaces of the block is significantly affected by changes in block spacing and underscour depth. The experimental results show that for the same underscour gap, lift, and drag coefficients decreased with increasing space between the two blocks. In addition, if the underscour gap is increased with the same horizontal spacing, the lift coefficient increases and drag coefficient decreases. These results demonstrate that wider spacing enhance stability of the frontal block in the underscour condition. Moreover, analysis of block stability with changing underscouring and horizontal spacing shows that bed protection blocks seem to have more stability when the gap between two bed protection blocks was equal to height of the blocks. The stability of a river-bed protection block in a flood event is affected by underscoring the block. The effects of block spacing on block stability were investigated in different underscour conditions. Three blocks were placed at the bottom in a flume, and the underscour depth of the frontal block and distance between the first and second blocks were changed. Drag and lift forces were measured directly in each experimental condition. Particle Image Velocimetry (PIV) was used to visualize the flow and eva- luate the streamline pattern. around the block to understand the pressure distribution show that the pressure distribution on the rear and bottom surfaces of the block is significantly affected by changes in block spacing and underscour depth. The experimental results show that for the same underscoring gap, lift, and drag coefficients decreased with increasing space between the two blocks. In addition, if the underscoring gap is increased with the same horizon Moreover, the analysis of block stability with changing underscouring and horizontal spacing shows that bed protection blocks seem to have more stability when the gap between two bed protection blocks was equal to height of the blocks.