The evaluation of early age properties of concrete is critical for ensuring construction quality. This paper presents a new method for monitoring the early age properties of cementitious material using ultrasonic guided waves in a rebar. Longitudinal mode L(0,1) wave in rebar is excited by an EMAT sensor and received by a P-wave ultrasonic transducer. Dispersion curves are developed to choose appropriate frequency with low attenuation and dispersion. Continuous measurements during cement hydration are used to monitor the longitudinal wave attenuation resulted from the leakage from the rebar to the surrounding concrete. The evolution of the p-wave and shear wave velocity is also measured at the same time. Experiment attenuation is verified by the calculated attenuation in both time domain and frequency domain based on ultrasonic wave velocity measurements. Experiments were performed on three cement paste samples and three mortar samples. The results indicate that attenuation is linearly related to the shear wave velocity and shear wave velocity is linearly related to the penetration resistance (ASTM C403) in logarithmic scale. Mechanical properties and hardening process can be monitored using the two linear relationships.