As inorganic and organic mercury species are the most harmful substances to human health, detection of mercury species in aquatic environments and humans is important. Particularly, methylmercury species are highly toxic than inorganic mercury species because methylmercury species have high ability to penetrate lipid membranes of various organs and accumulate easily in endocrine system, kidneys, and brain, leading to severe toxicities to human body. Selective fluorescent detection of CH₃Hg(I) over Hg(II) is rare. Herein, we synthesized five fluorescent mono-peptidyl probes (1-5) bearing tryptophan amino acid (Trp) and N,N-dimethylamine-7-sulfonyl-2,1,3-benzoxadiaole (ABD) fluorophore which replaced the dimethylamino substituents of the ABD fluorophore with different size of cyclic amine. Trp played an important role of quencher for some fluorophores through photoinduced electron transfer (PET) process as well as a ligand for mercury species. Therefore, mechanism of this sensor involves binding with mercury species and PET process was blocked and enhancement in fluorescence intensity was observed. Increasing the size of cyclic ring increase the hydrophobicity of the probe which enhance lipid membrane penetration capability. Since CH₃Hg(I) is more hydrophobic compared to Hg(II), CH₃Hg(I) interacts easily with a hydrophobic probe in micelle system and shows turn-on response, also increasing ring size required less CH₃Hg(I) for saturation. Further, similar kind of distinct emission intensity changes was observed in presence of Hg(II), smaller the ring size shows enhancement in emission intensity but bigger the ring size shows no change of emission intensity but slight red shift. 1-5 in micelles exhibited remarkable detection ability for CH₃Hg(I) such as ultrafast response (<30 s), while 4 and 5 showed high sensitivity and selectivity for CH₃Hg(I). Fluorescent probe with micelles provided a selective detection of CH₃Hg(I) over Hg(II) or distinguished between CH₃Hg(I) and Hg(II) by fluorescent response types.