The influences of cold exhaust gas recirculation (EGR) and cetane number (CN) improver-di-tertiary butyl peroxide (DTBP) on the combustion characteristics and emissions of the Homogenous Charge Compression Ignition (HCCI) combustion engines fueled with PRF90 fuels (a mixture of 90% iso-octane and 10% n-heptane by volume) were studied in this paper. The experiments were performed using 0%, 1%, 2%, 3%, and 4% (by volume) DTBP-PRF90 blends. The experimental results show that the operating range was remarkably extended to lower temperature and lower engine load with the DTBP addition in PRFRON90 fuel. With the improvement of HCCI combustion at the lower temperature and light load by CN improver, the combustion phase of higher temperature and large load can be retarded to the top dead center (TDC) by appropriated EGR rate. As a result, the indicated thermal efficiency and indicated specific fuel consumption (ISFC) were improved.