Sequestration of Carbon Dioxide (CO2) into saline aquifers has been proposed as one of the most practical options of all geological sequestration possibilities. When saline aquifers are to be used to sequester CO2 for long periods, it will be necessary to monitor the migration and diffusion of CO2 in those reservoirs. Monitoring of geological sequestration has been identified as one of the highest priority needs in several recent international conferences on greenhouse gas control technologies. Monitoring is necessary to confirm the containment of CO2, to assess leakage paths, and to gain understanding of interactions between CO 2, the rock-forming minerals, and formation fluids. Recently CO 2 monitoring has moved to next stage for the purpose of leakage detection and quantification of CO2 stored in reservoirs. What kinds of monitoring methods we could use and do the methods have sufficient resolution and detection levels need to be addressed urgently. Seismic surveys provide the most attractive approach for obtaining the spatial coverage required for mapping the location and movement of CO2 in the subsurface. However, from the first Japanese pilot project, time-lapse sonic logging results showed P-wave velocity becomes less sensitive when the CO2 saturation up to 20%, while resistivity kept increasing with increase in CO2 saturation. This paper describes the results of P-wave velocity and resistivity measurements when injecting CO2 into water-saturated porous sandstones at laboratory and the results of comparison between P-wave velocity and resistivity changes obtained from both laboratory- and field-scales.