A new highly sensitive strain sensor has been developed by applying the strain-induced change of the electrical conductivity of multi-walled carbon nanotubes (MWCNTs). The electric conductivity of MWCNTs changes drastically under uni-axial strain because of the drastic change of their electronic band gap. Therefore, the local strain distribution can be detected by measuring the change of the electric resistance of MWCNTs under strain. In order to design a new sensor using MWCNTs, a method for controlling the shape of the MWCNTs was developed by applying a chemical vapor deposition (CVD) technique. It was found that the shape of the grown MWCNTs can be controlled by changing the average thickness of the catalyst layer and the growth temperature. The electrical resistance of the grown MWCNT bundle changed almost linearly with the applied uniaxial compressive strain, and obtained maximum strain sensitivity was about 10%/1000-ustrain (gauge factor: 100). A two-dimensional strain sensor, which consisted of area-arrayed fine bundles of MWCNTs, was developed by using MEMS technology. Under the application of compressive strain, the electric resistance was confirmed to increase almost linearly with the applied strain.