A quantitative estimate using daily circulation data for 55 years reveals that the potential predictability of monthly mean summer monsoon climate (a ratio between 'external' and 'internal' interannual variances) has decreased by almost a factor of two during the decades of 1980's and 1990's as compared to that during 1950's and 1960's associated with the major interdecadal transition of climate in mid-1970's. During the same period, however, the potential predictability of the summer climate over the central and eastern tropical Pacific has increased by a factor of two. The decrease in potential predictability of monsoon is partly due to a large decrease in 'external' variability during the recent decades as a result of interdecadal modulation of the mean monsoon circulation and partly due to 'internal' variability remaining relatively high. It is shown that the decrease in 'internal' variability due to decrease in intraseasonal activity during this period is rather small and can not compensate the decrease is 'external' variability resulting in significant decrease in monsoon predictability.