A memory structure including a SOI substrate, a first transistor, a second transistor, an isolation structure and a capacitor is provided. The SOI substrate includes a silicon base, a dielectric layer and a silicon layer. The first transistor and the second transistor are disposed on the silicon layer. The isolation structure is disposed in the silicon layer between the first transistor and the second transistor. The capacitor is disposed between the first transistor and the second transistor. The capacitor includes a body portion, a first extension portion, a second extension portion and a third extension portion. The first extension portion extends from the body portion to a source/drain region of the first transistor. The second extension portion extends from the body portion to a source/drain region of the second transistor. The third extension portion extends from the body portion, penetrates through the isolation structure and extends into the dielectric layer.