Rock bolts are widely used in mining and geotechnical engineering as ground anchorage. The supporting capacity of grouted rock bolts depends greatly on the grout quality in rocks. Measurement of the grout quality in the field is an outstanding issue. In an effort to develop a non-destructive method for estimating the grout quality of grouted rock bolts, the characteristics of guided ultrasonic waves are investigated in this paper. Particular attention is paid to the effects of grout quality on group wave velocity and attenuation. Issues associated to grout quality and bolt failure are also discussed. To simulate the in-situ condition, several specimens were prepared using 20 mm diameter rebar. Each specimen was grouted in a 200 mm diameter concrete cylinder, which was designed with different compressive strength. A large number of tests were conducted on these specimens using ultrasonic waves with frequencies from 10 to 100 kHz. The effects of air content and compressive strength of the grout on attenuation and group velocity of ultrasonic waves in the grouted rock bolts were studied. The results showed large influence from the grout strength and air content and demonstrated the potential for using ultrasonic waves to test grout quality.