Abstract: A die assembly (20) for riveting includes an upper die (200), a base die (230), a pair of clamps (270), and a pair of springs (280). The upper die includes a fixing block (202), and a pair of pressing protrusion (204) at opposite sides of the fixing block and having first slanted surfaces (206). The base die includes a sizing block (232) to position a workpiece. Two supporting blocks (234) are formed at opposite sides of the sizing block, each defining a through hole for accommodating the clamp. Two positioning blocks (240) are at opposite sides of the supporting blocks. Each clamp includes an end block (276) having a second slanted surface (278) corresponding to the first slanted surface of the pressing protrusion, and a connecting portion (272) slidable in the through hole of the supporting block. The springs connect the clamps to the positioning blocks.