Methylated cytosine is proved to have an important role as an epigenetic signal in gene regulation and is often referred to "the fifth base of DNA". A comprehensive understanding of the electronic excited state relaxation in cytosine and its methylated derivatives is crucial for revealing UV-induced photodamage to the biological genome. Because of the existence of multiple closely lying "bright" and "dark" excited states, the decay pathways in these DNA nucleosides are the most complex and the least understood so far. In this study, femtosecond transient absorption with different excitation wavelengths (240-296 nm) was used to study the relaxation of excited electronic states of 5-methylcytosine (SmC) and 2'-deoxy-S-methylcytidine (SmdCyd) in phosphate buffered aqueous solution and in acetonitrile solution. Two distinct nonradiative decay channels were directly observed. The first one is a several picosecond internal conversion channel that involves two bright pi pi* states (pi pi*(2) and pi pi*(1)) when pi pi*(2) state is initially populated. The second channel contains the lower energy pi pi*(1) state and a so far experimental unidentified long-lived state which exhibits a several nanosecond lifetime. The long-lived state can only be accessed by the initially excited pi pi*(1) state. Inspired by this new discovery in SmC and SmdCyd, we revisited the decay of excited state of 2'-deoxycytidine (dCyd), revealing very similar decay pathways. Additionally, a well-known dark n(O)pi* state (carbonyl lone pair) with,similar to 30 ps lifetime is present in both decay channels in dCyd. With our detailed experimental results, we successfully reconcile the long history debate of cytosine excited state relaxation mechanism by pointing out that the reason for the complex dynamics under traditional 266 nm excitation is mixed signals from the above-mentioned two distinct decay pathways. Our findings lead to a dramatically different and new picture of electronic energy relaxation in SmdCyd/dCyd and could help to understand photostability as well as UV-induced photodamage of these nucleotides and related DNAs.