Guanidinium cation and urea are often used as protein denaturants because of their strong hydrogen-bond ability. A deep eutectic solvent (DES) composed of guanidine hydrochloride (GuHCl, Tm = 458 K) and urea (U, Tm = 407 K) with a molar ratio of [U]/[GuHCl] = 2 is a liquid medium that has the potential to denature proteins and dissolve biomaterials. However, heating is essential when using 2U-GuHCl as a liquid solvent. This is because the eutectic temperature of this DES is 331 K, which is higher than room temperature. This heating is a serious problem when dissolving solutes with low thermal stability. We focused on two urea derivatives, N-methylurea (Um, Tm = 378 K) and N-hydroxyurea (Uh, Tm = 414 K), to obtain GuHCl-based DESs in liquid state at room temperature. Since all of the GuHCl-based DESs containing Um molecules were fluid at room temperature, the Um molecule is a candidate hydrogen bond donor for fluidizing GuHCl-based DESs. This paper presents experimental results of binary and ternary DESs composed of GuHCl, U, Um, and Uh. In addition, preparation methods for general DESs are outlined.