A high intensity discharge lamp has an arc tube, which is filled with substances for gas discharge. The arc tube includes a bulbous portion. Surrounding the arc tube there is a tubular envelope. The envelope has an enlarged portion, which is similar to the bulbous portion. The distance between the envelope and the outer wall of the arc tube is uniform thereby resulting in uniform temperature distribution on the wall of the arc tube, improved thermal stability, improved luminous and electrical parameters. The lamp reduces the light refraction which occurs between the inner surface of the envelope and the outer surface of the arc tube and also between the two interface of the envelope wall thereby emitting more light and reducing the occurrence of glare.