We present experimental results on the sub-Doppler Rydberg spectroscopy of potassium in a hot cell and cold atoms, using two counterpropagating laser beams of 405 and 980 nm as an inverted ladder-type excitation configuration (4S 1/2-5P 3/2-nS 1/2 and nD 3/2,5/2). Such an inverted ladder-type scheme is predicted to be without the sub-Doppler electromagnetically induced transparency feature in a thermal ensemble under the weak-probe approximation. Instead, we utilize a strong probe field and successfully observe a transparency window with a width narrower than 50 MHz. Our all-order numerical simulation is in satisfactory agreement with the experimental results. This narrow linewidth allows us to measure the energy levels of the Rydberg levels from n = 20-70 with improved accuracy. The deduced ionization energy agrees with the previous measurements. Furthermore, the two-photon Rydberg excitation scheme was applied to the cold ensembles to study the ground-state atoms population decrease in the magneto-optical trap for various Rydberg states. Our experimental observations suggested two distinct regimes of the trap losses under different probe detuning conditions. While the far off-resonance case (p 0) can be described by the picture of dressed atom, the on-resonance case (p ∼ 0) reveals more interesting results. The higher Rydberg states suffer larger trap loss. Besides, even with similar level energies, the excitation to nD states result in faster escape of the ground-state atom from trap than nearby nS states.