A sapphire-loaded cavity resonator with YBa₂Cu₃O_7- (YBCO) endplates and the TE₀₁ mode resonant frequency (f₀) of about 19 GHz at 77 K was prepared and its microwave properties were investigated at low temperatures. Unloaded Q (Q₀) of the TE₀₁ mode resonator appeared to be more than 1000 000, 600 000 and about 300 000 at 30 K, 60 K and 77 K, respectively, at f₀~19.56 GHz with the sapphire rod sandwiched between two YBCO endplates. Q₀ appeared to decrease as the gap distance (s) between the sapphire rod and the top endplate increased for the resonator with YBCO endplates at 30 K, which was in contrast with the increase of Q₀ for increasing s for the cavity resonator with copper endplates. Microwave losses from the surrounding copper walls appeared to reduce Q₀ of the TE₀₁ mode resonator with YBCO endplates significantly at low temperatures. Experimental values of the TE₀₁ mode resonant frequencies appeared to agree well with both the calculated values from analytic expressions and the simulated values from a commercial software within 1.3%. Variations in f₀ upon the gap distance were more than 4.8% between s = 0 mm and 1 mm, with Q₀ of the TE₀₁ mode resonator about 770 000, 530 000 and 350 000 at s = 1 mm at 30 K, 60 K and 77 K, respectively.