Cyprodinil, (4-cyclopropyl-6-methyl-pyrimidin-2-yl)-phenyl-amine is a fungicide of broad spectrum. Usually it is applied to control a range of various pathogens. Post harvest rests of cyprodinil on fruits and vegetables have irritant influence on human organism. Cyprodinil is resistant to action of acids and basis; however it can be rapidly degraded in water when exposed to UV light with a half live of 13.5 days [1]. In this connection the aim of our DFT study (with the use of GAUSSIAN 09 program unit [2]) consists in definition of possible mechanism of its photo-transformation. First of all, it is shown that the possible interactions between a molecule of cyprodinil and the ions H+, OH- or water itself do not lead to any transformation of cyprodinil molecular skeleton. However, calculations show that cyprodinil in its excited state is capable to further structural transformations (isomerization) forming the photoproduct (see the Figure). One can see that for the ground electronic state (S=0) its tautomeric form I (a) is more stable than the form II (b), while in the excited electronic state (S=1) the more energetically preferable form is the tautomer II (e). The jumping of the proton in (e) from the nitrogen atom to one of the ‘banana’ bonds of the cyclopropyl ring leads to its opening of three-membered ring and to the following formation of the new five-membered cycle including the above nitrogen atom (see the excited structure (f) of the Photoproduct). The release of the excitation energy leads to the Photoproduct in itsground electronic state (c) whose energy is lower than that of Tautomers I and II. The experimental results of the work [3] confirm the formation of this pfotoproduct.Thus, unlike the case of fungicide vinclozolin whose proton-induced decay was revealed by us earlier [4], the main role in the photo-transformation of cyprodinil plays the abovementioned change of its tautomeric form under sunlight action.