Abstract: There exists a suite of gray rhyolitic rocks in Sonid Left Banner of Inner-Mongolia. According to the petrological features, these volcanic rocks are considered to be a part of Upper Cretaceous Baiyingaolao Formation. With the LA-ICP-MS dating analysis, however, the authors obtained four U-Pb ages, i.e., 305.1±1.1Ma, 304.5±1.1Ma, 305.7±1.9Ma, and 303.6±1.1Ma, indicating that they belong to the Upper Carboniferous Baoligaomiao Formation. The result achieved by the authors has guiding significance for stratigraphic division and determination of magma evolution.