A lamp comprising: a heat dissipater (1, 1', 1") including a base and an engaging member, with the base including a fixing portion, with the engaging member mounted to an outer periphery of the base; a lighting element mounted to the base of the heat dissipater (1, 1' 1"); a housing (3,3') engaged with the engaging member of the heat dissipater (1, 1', 1"), with the housing (3, 3') and the heat dissipater (1, 1', 1") together defining a compartment, and with the housing (3, 3') including a first end and a second end opposite to the first end, with an opening formed at the first end thereof; a cooling fan mounted to the fixing portion of the base; and a circuit board mounted in the compartment and electrically connected to the lighting element and the cooling fan, with the circuit board electrically connected to an electrically conductive member having an end extending out from the housing (3, 3'), the heat dissipater (1, 1', 1") is arranged at the opening, wherein the lamp is characterized in that the base includes a first air guiding portion, a second air guiding portion (121, 34) is formed between the engaging member and the base, the first air guiding portion and the second air guiding portion (121, 32) are arranged at the opening, the compartment communicates with the first and second air guiding portions (111, 121, 34), the cooling fan is adapted for driving air currents into the compartment and the first air guiding portion or the second air guiding portion (121, 32), and the cooling fan is also adapted for guiding the air currents out of the compartment and the second air guiding portion (121, 32) or the first air guiding portion, respectively. a heat dissipater (1, 1', 1") including a base and an engaging member, with the base including a fixing portion, with the engaging member mounted to an outer periphery of the base; a lighting element mounted to the base of the heat dissipater (1, 1' 1"); a housing (3,3') engaged with the engaging member of the heat dissipater (1, 1', 1"), with the housing (3, 3') and the heat dissipater (1, 1', 1") together defining a compartment, and with the housing (3, 3') including a first end and a second end opposite to the first end, with an opening formed at the first end thereof; a cooling fan mounted to the fixing portion of the base; and a circuit board mounted in the compartment and electrically connected to the lighting element and the cooling fan, with the circuit board electrically connected to an electrically conductive member having an end extending out from the housing (3, 3'), the heat dissipater (1, 1', 1") is arranged at the opening, wherein the lamp is characterized in that the base includes a first air guiding portion, a second air guiding portion (121, 34) is formed between the engaging member and the base, the first air guiding portion and the second air guiding portion (121, 32) are arranged at the opening, the compartment communicates with the first and second air guiding portions (111, 121, 34), the cooling fan is adapted for driving air currents into the compartment and the first air guiding portion or the second air guiding portion (121, 32), and the cooling fan is also adapted for guiding the air currents out of the compartment and the second air guiding portion (121, 32) or the first air guiding portion, respectively.