The history of the eleven-year solar cycle over the last 1200 years, including the periods of the Maunder Minimum (1645-1715AD) and the Medieval Maximum (around the 9th-12th century), will be presented. The transitions of solar cycle have been investigated by measuring the carbon-14 content in tree-rings, which is one of the indices of solar magnetic activity. We find that the actual length of the eleven-year solar cycle were ~14 years and ~9 years during the Maunder Minimum and the Medieval Maximum Period, respectively. The length of climate decadal cycle seems to have lengthened/shortened accordingly. The detailed feature of climate variation during the Maunder Minimum and the possible mechanism of solar forcing at the decadal time scale are also discussed.