Beam diagnostic for electron bunch length using spectrum analysis of multimode terahertz (THz) -wave on the order of 0.1 THz was investigated. The multimode THz-wave was generated by coherent Cherenkov radiation (CCR) using hollow dielectric tubes and femtosecond/picosecond electron bunches. The spectra of multimode THz-wave depended on electron bunch length, which was measured by a streak camera, resulting in a possibility of a new beam diagnostic based on CCR and a bunch form factor. Copyright © 2012 by the respective authors.