This study was performed to develop a more simple and convenient heat treatment process for increasing heating temperature than the traditional 9-times steaming process for Rehmanniae radix Preparata (R. radix P.). The R. radix was heated to various temperatures (110, 120, 130, 140 and $150^{circ}C$) for different lengths of time (1, 2, 3, 4 and 5 hr). Chromaticity, free sugar, amino acid, catalpol and 5-HMF (5-hydroxy-methyl-2-furaldehyde) content of water extracts of R. radix P. were analysed. With increasing heating temperature and time, the L, a and b-value of samples gradually decreased. Fructose and glucose contents were increased to $120^{circ}C$ for 5 hr (5.21 and 6.67%, respectively) with no changes afterward. Sucrose content was not detected after $130^{circ}C$. Total amino acid was increased to $140^{circ}C$ and decreased afterward. The maximum total amino acid content was 4,172 mg/100 g at $140^{circ}C$ for 2 hr (R. radix L.: 3,029 mg/100 g). The catalpol was decreased to $130^{circ}C$ for 1 hr and not detected afterward. The 5-HMF content increased with increasing heating temperature and time. The maximum content of 5.HMF was 2.5% in $150^{circ}C$ for 5 hr. Hence, it can be suggested that the heat treatment at $130^{circ}C$ for 2 hr or more is the most optimum processing conditions, instead of the traditional 9.times steaming process.