Tropospheric ozone (O3) is a trace gas playing important roles in atmospheric chemistry, air quality and climate change. In contrast to North America and Europe, long-term measurements of surface O3are very limited in China. We compile available O3observations at Mt. Tai - the highest mountain over the North China Plain - during 2003-2015 and analyze the decadal change of O3and its sources. A linear regression analysis shows that summertime O3measured at Mt. Tai has increased significantly by 1.7ppbv yr-1for June and 2.1ppbv yr-1for the July-August average. The observed increase is supported by a global chemistry-climate model hindcast (GFDL-AM3) with O3precursor emissions varying from year to year over 1980-2014. Analysis of satellite data indicates that the O3increase was mainly due to the increased emissions of O3precursors, in particular volatile organic compounds (VOCs). An important finding is that the emissions of nitrogen oxides (NOx) have diminished since 2011, but the increase of VOCs appears to have enhanced the ozone production efficiency and contributed to the observed O3increase in central eastern China. We present evidence that controlling NOxalone, in the absence of VOC controls, is not sufficient to reduce regional O3levels in North China in a short period.