This article presents a simple design of a coaxial dual-band cavity filter and diplexer using stub-loaded resonators (SLRs). Based on the resonance characteristics of a coaxial SLR, the first two resonant frequencies can be tuned over a wide range by adjusting their structural parameters. Controllable external quality factors can be acquired by using the modified tapping structure for the two designated passbands. A coupling window and a coupling pin are proposed to obtain the expected coupling coefficients in the two bands. To demonstrate this concept, a third-order dual-band bandpass filter and an eight-pole diplexer are designed and fabricated. The measured results agree well with the full-wave simulation results.