The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA-H2O) in the S1 state have been measured by picosecond pump-probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S1 - S0 origin is 8 – 9 ps. On the other hand, the lifetime of OMpCA-H2O complex at the origin is 930 ps, which is ~100 times longer than that. Furthermore, in the complex the S1 lifetime shows rapid decrease at an energy of ~200 cm-1 above the origin and finally becomes as short as 9 ps at ~500 cm-1. Theoretical calculations with symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by noradiative decay dynamics involving trans →cis isomerization. That is both OMpCA and OMpCA-H2O in the S1 sate decay due to the trans→cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S1 surface, while in OMpCA-H2O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA-H2O is in good agreement with that observed experimentally.