A lamp string includes bulb mounting units arranged electrically in series with each other, and light-emitting units, each provided with two conductive leads. Each bulb mounting unit includes: a receptacle formed with angularly displaced first and second inner shoulders, the first inner shoulder having a shorter depth than the second inner shoulder from a top open end of the receptacle; a mounting seat mounted in the receptacle and including a flange formed with an axially extending protrusion, the flange having a portion seated on the first inner shoulder, the axially extending protrusion being seated on the second inner shoulder; and first and second conductive contacts contacting the leads of a respective light-emitting unit.