One of the erosion control options in the downstream of the overflows is to use crushed stone to reduce the excess output energy and to minimize the amount of erosion and scouring in the downstream of the overflow. In this research, experiments were conducted to check the stability of the rock in the downstream of the spillway by changing the threshold angle. For this purpose, a triangular cup overflow with four different threshold angles was made of fiberglass. Experiments were carried out using a piled stone density with four different diameters in clear water. In each experiment, the flow depth was measured under the movement threshold conditions and then the stability number was calculated using the obtained data. The results of this research showed that the highest stability number related to the overflow with a threshold of 45 degrees and the lowest stability number was observed in the overflow with a threshold of 15 degrees. Also, the relative diameter of the aggregates is also an effective factor in the stability of the aggregates, and the stability number at the threshold of movement in the four angles of the triangular overflow threshold decreases with the increase of the relative diameter of the aggregates. In order to provide a relationship to estimate the relative diameter of the checkered stone, the variables of the investigation and the correlation coefficient of the results obtained from this relationship with the laboratory results for the triangular overflow were obtained as 0.85.