The interaction between intense femtosecond laser and Argon atomic clusters is analyzed using the extreme ultraviolet(XUV) spectroscopy. The laser system has the capability of generating 20-100 fs light pulse with energy of 30 mJ(peak intensity ~10^16 W/㎠, and the atomic clusters are formed using a cryogenically cooled gas jet. The size of cluster is varied by controlling the temperature of gas jet. By focusing the laser pulse into the atomic clusters, soft X-ray emission(40-180 Å) is observed with a flat-field XUV spectrometer. As the temperature of gas jet is lowered, the significant enhancement of spectral lines and the occurrence of the spectral lines from highly charged ions ($Ar^{9+}$, $Ar^{10+}$) are observed, which indicates the existence of effective heating mechanisms in cluster plasma of near-solid-density. Also the effect of pulsewidth is investigated.