X‐ray polarization spectroscopy was studied to derive directly the velocity distribution function (VDF) of hot electrons propagating in plasma created with a high intensity laser pulse. Polarization measurement was made at around 1018 W/cm2 using a laser pulse (∼10 J in ∼1 ps) from Alise facility at CEA/CESTA. Chlorinated triple‐layer targets were irradiated, and Cl Heα line was observed with an x‐ray polarization spectrometer. Polarization degrees were measured as a function of the target overcoat thickness, corresponding to the depth along pre‐formed plasma. It is found that the polarization is weakly negative for thin coating, but becomes positive, and finally zero for thick coating. This result is consistent with predictions made with a time‐dependent atomic kinetics code developed to gain an insight into the generation of polarized Cl Heα radiation. The de‐polarization on the surface is attributed to excessive bulk electron temperature and that in the deep region to elastic‐scattering processes by th...