An LED lamp having a heat-dissipating mechanism has a top casing, a heat-conducting tube and a bottom casing. The top casing has a foundation board, a lamp holder and multiple fins. The foundation board has a first tube guide formed around and protruding downwardly from a perimeter of the foundation board and is made of a metal material. The lamp holder is hollow and centrally mounted through the top casing, and has a second tube guide formed on a bottom thereof. The fins are radially formed on a top of the foundation board and around the lamp holder. The heat-conducting tube is mounted against one side of the first tube guide and within the second tube guide. The bottom casing is securely combined with the top casing. Besides the fins, the heat-conducting tube enlarges the contact area with the top casing, thereby enhancing the heat-dissipating efficiency.