Few studies have investigated the relationship between cortisol diurnal rhythm and cognitive function in healthy young adults, especially for emotional memory. To address this deficiency, this study examined the effect of diurnal cortisol slope (DCS) and heart rate variability (HRV) on emotional memory. Participants included healthy volunteers (44 men and 23 women; mean age 20.60 yrs). Participants were shown emotionally arousing slides and were asked to return to the laboratory one week later where they were given a "surprise" memory test to examine their emotional memory retention. Participants were asked to collect saliva samples at four time points (08:00, 11:00, 15:00, and 20:00) on the experimental days; these samples were used to calculate the DCS. Moreover, HRV was measured during the experiment. The multiple linear regression analysis revealed that declarative memory ability, sleep duration, and the DCS were the final significant determinants for emotional memory enhancement (B = -20.41, 0.05, -48.20, ps < 0.05), with participants having flatter cortisol slopes showing reduced or absent emotional memory enhancement. These findings are discussed in reference to the possible effects of diurnal rhythm mechanisms of the hypothalamus-pituitary-adrenal axis and the autonomic nervous system on emotional memory.