An onboard lamp fixture for a vehicle includes a hollow conical body. The conical body is divided into a top portion having multiple flanges formed on an outer periphery of the top portion and multiple indentations each defined between two adjacent flanges, a middle portion having multiple intermittently formed threads on an outer periphery of the middle portion and annular gaps each defined between two adjacent threads and a bottom portion. After the conical body is extended through a through hole, a bottom face defining the annular gap of each thread is able to engage with an outer periphery defining the through hole and the flanges are able to securely engage with an inner periphery defining the through hole to secure the conical body.