Ultrafast UV-vis/IR spectroscopies were used to study the photochemistry of a vinyl diazo ester PhCH=CHCN2CO2CH3 (1) in solution. The results indicate that singlet styrylcarbomethoxy carbene ((1)2) is produced from the excited state of diazo precursor (1*). It is concluded that vinyl singlet carbene ((1)2) undergoes an intramolecular cyclopropenation reaction to produce the cyclopropene product (3), and undergoes intersystem crossing to ground triplet carbene ((3)2). The predictions of DFT calculations are consistent with the observations.