Height adjusting mechanism for an armrest

A height-adjusting mechanism for armrest includes a slider movably mounted in a slide way of a sleeve connected at a lower end to a seat of an armchair. A retaining block horizontally extends through a transverse hole on the slider and into one of many vertically spaced locating holes on the slide way to hold the slider at a certain height relative to the sleeve. A long plate having a top handle is attached to one side of the slider and could be pulled upward and elastically return to an original position when being released. When the long plate is pulled upward, it drives the retaining block to disengage from the locating hole and allows the slider to freely move up and down. And, when the long plate is released, the retaining block is driven to engage into another locating hole to hold the slider to a new height.