From 1955 to 2000, China has experienced a decrease in the number of frost days, while the length of the frost-free season between the last spring freeze and the first fall frost has increased. Three distinct regimes can be detected in the time series: up to about 1973, the annual number of frost days was about 2 d higher than the 1961–1990 average; from 1973 to 1985, the annual number of frost days held close to that average with remarkably little interannual variability; and after 1985, the annual number of frost days decreased rapidly with distinct reversal around 1992. The dates of first and last frost show two patterns: before 1980, these dates fluctuated around the 1961–1990 average, but after 1980 (and especially from 1993) the frost-free season was rapidly lengthened. The numbers of frost days are highly correlated with minimum temperature (Tmin) in north China in spring and fall; while in south China frost dates correlate with minimum temperatures in winter. Generally, the seasonal relationships between Tmin and frost days are significant in both the temporal and spatial domains when seasonal average Tmin falls within a range of ±10°C. Analyzing annual and seasonal influences on the number of frost days, we find that water vapor plays a significant role. Regionally, the greater influence on the length of the frost-free season in south China has been the delayed onset of the autumn frost, while in north China the spring and autumn dates each have a comparable influence on the length of the frost-free season. The initial lengthening of the frost-free season lagged about 10 years behind the rapid increase in daily minimum temperatures, while the decrease in the annual number of frost days lagged by about 15 years.