Theoretical studies of climatology during the 20th century was reviewed. Ten items of the most important achievements were outlined as following:①NAO,NPO and SO,②circulation index,③30-day mean circulation,④Walker circulation,⑤greenhouse effect,⑥prediction of monthly mean circulation,⑦prediction of ENSO,⑧delayed oscillator,⑨thermohaline circulation, and10prediction of seasonal mean circulation. Some problems which may become the hot topics in early 21st century, such as
climatic prediction, East Asian monsoon, ENSO, interdecadal variability of climate, and detection of greenhouse effect,are discussed briefly.