To explore the effect of the addition of ethanol (E) on the combustion behavior of biodiesel/n-heptane (BH) blends, autoignition characteristics of the BHE blends were studied in two experimental systems: a modified cooperative fuel research (CFR) engine and a constant-volume combustion chamber (CID 510) used for rating the derived cetane number of fuels. The observations of ignition behavior include the critical compression ratio and heat release profile, which are assessed using the CFR engine. The equivalence ratio is 0.25 and 0.45, respectively, while the physical and chemical ignition delays are measured by the CID 510 under a wide range of air temperatures and oxygen dilution levels. With the addition of the ethanol, the critical compression ratio increases, which indicates that the reactivity decreases. According to the heat profiles, because of the complex composition of the blend, the onset of the high temperature heat release (HTHR) and low temperature heat release (LTHR) did not vary linearly with ethanol concentration, and the onset of LTHR of BHE15 and BHE20 is very close at both equivalence ratios at the same compression ratio (5.2). This is consistent with almost the same cetane number of BHE15 and BHE20. With the increase of ethanol in the blend, the physical ignition delay at different temperatures was BHE20 > BHEIS > BHE10 > BHES > BH. In addition, the chemical ignition delay increased with the addition of ethanol except for BHES, which showed negative temperature coefficient (NTC) behavior and displayed a shorter chemical ignition delay than that of the BH blend at 853.15 K. The physical ignition delay for BH, BHES, and BHE10 increased slightly with oxygen dilution. Moreover, the chemical ignition delay increased sharply with increasing exhaust gas recirculation (EGR). Higher addition of ethanol results in higher chemical ignition delay. The heat release profiles for the blends at different temperatures and EGR levels showed a decrease in reactivity.