High-harmonic-seeded x-ray laser became an important issue in x-ray laser development due to the possibility to obtain a highly coherent and polarized soft x-ray source. We performed theoretical investigations into amplification of high harmonic pulses in an x-ray lasing medium by using a model based on Maxwell-Bloch equations. From the theoretical works, we analyze characteristics of energy extraction and temporal profile of output pulse. In addition, preliminary experimental results and ongoing experiments related the harmonic-seeded x-ray lasers are reported.