This paper reports the principle and the experimenlal vesults of the ring laser magnetic sensor for weak field (RLMS) for the first time. Theoretical analysis indicates that the ELMS suseceptibility mainly depends on the merit coeffecient of magneto-optical material. In our experiments a terbium-silicate glass of 20 mm length is used as a Faraday cell, a susceptibility of 2.9 ×10~(-8) Oe/Hz (0.0023A/m·Hz) and a random shift of less than 2.3Hz during 9 hour have been obtained.