Spark assistance for homogeneous charge compression ignition (HCCI) can control combustion phasing, improve thermal efficiency, and reduce emissions in gasoline engines. As the characteristics of flame propagation determine the control authority of ignition timing, it is important and necessary to investigate pressure dependence of flame speed in the lean-premixed mixture relative to engine operating conditions. Experimental study in an optical rapid compression machine (RCM) and simulation work were carried out using two fuels comprising n-heptane/iso-octane/ethanol with varied octane sensitivity (S). The effective pressure ranged from 10 to 35 bar, temperature from 715 to 860 K, and equivalence ratios between 0.3 and 0.7 to cover the region of lean flammability limits of low and high S fuels with ethanol blended. Based on pressure profiles, flame speed extracted from images, and sensitivity analysis of flame speed, the dependence of flame speed on the effective pressure in low and high S fuels was discovered and the fundamental mechanism behind this phenomena became to be understood in the negative temperature coefficient (NTC) and non-NTC regions, respectively. In the studied temperature conditions, the flame speed of high S fuel has stronger dependence on the pressure than that of low S fuel does. In the NTC region, this phenomenon is attributed to the dependence of H radical concentration on pressure in the unburned mixture and flame structure. In the non-NTC region, promoting effect of dominant reactions varied with pressure can significantly influence pressure dependence of flame speed. Although quite limited data of laminar burning velocity for studied fuels were obtained in high pressures (>15 bar), the trend of flame speed's dependence on pressure was well predicted by two models with different but well-accepted core mechanisms, showing consistent results with the experimental ones in the RCM. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.