Hydrothermal carbonization (HTC) is an effective pretreatment technology for converting municipal solid waste (MSW) into homogenized, energy-dense, and carbon-rich hydrochars with low energy consumption. In this study, heavy metals and free polycyclic aromatic hydrocarbons (PAHs) in MSW and corresponding hydrochars were investigated. The results showed that the hydrochar yield decreased with an increasing temperature from 160 to 260 degrees C. Heavy metal contents, including Cr, Cd, Hg, and Zn, in the hydrochars were lower than those in MSW, while Pb, As, Ni, and Cu showed an accumulation in the hydrochars at most temperatures. In addition, the Toxicity Characteristic Leaching Procedure test showed that the contents of heavy metals in leachates were all lower than the United States Environmental Protection Agency (U.S. EPA) limits. With regard to PAHs, total free PAH contents in the hydrochars were higher than those of MSW, except for the hydrochar obtained at 160 degrees C. The total PAHs in the hydrochar increased with the increase of the temperature from 160 to 240 degrees C and then significantly decreased with further increasing the temperature. The three-ring PAHs were dominant in the hydrochars, while for MSW, PAHs were mainly up to four-ring PAHs. The toxic equivalent quantity values of the hydrochars were higher than those Of MSW, except for the hydrochars obtained at 160 and 180 degrees C. The present study indicated that the significant reduction of heavy metals and PAHs in MSW could be achieved by HTC of MSW.