Over the past three decades, a generally increased air temperature occurred at most places in Inner Mongolia. Such a climate warming, which outpaced the global average warming rate, has been reported to significantly influence the vegetation phenology across the Inner Mongolia grasslands. However, there is less evidence from in situ observations to demonstrate the effect of climate warming on grass phenology. In this study, we analyzed the trends in both climatic and grassland phenological variations using the data from a 30‐year in situ observations at eight experimental sites in Inner Mongolia. We found that the most significant warming occurred during the growing season, while the temperature in nongrowing season did not show significant trends. Under this warming climate, two thirds of the observed phenologies (start, end, and length of the growing season) for different plant species across the eight sites did not show significant changes. Plant phenology exhibited divergent shifts in magnitudes and directions, depending on not only climate variation but also plant species (e.g., L. chinensis).