This study investigates the decadal variability of El Niño and La Niña occurrence in observations and examines that variability in a set of 20th Century climate simulations (20C3M) of coupled general circulation models (CGCMs) in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4).Wavelet analysis reveals that the observed frequency of El Niño events displays significant decadal variability with a period of about 12 years during 1920–1940, whereas the frequency of La Niña events shows significant decadal variations with a spectral peak at 16 years throughout the 20th century. Moreover, the frequencies of El Niño and La Niña events are influenced by different factors that are responsible for planetary teleconnections. The frequency of El Niño events is related the Atlantic Multidecadal Oscillation (AMO), while that of the La Niña events is associated with the Pacific Decadal Oscillation (PDO). Among the 15 IPCC AR4 CGCMs surveyed, csiro and miroc_medres CGCMs can reproduce the decadal variability of ENSO activity, and simulate partly its relationship with the Pacific and north Atlantic SSTa. These results will help us to further understand the important roles of the North Atlantic and North Pacific in ENSO variations.