Despite the spatio-temporal gap of 50 years, Baudelaire and Simmel have both mentioned the notion of modernity which puts into question the development of modern city. But, what did they mean by “modernity?” What was the problem that came with the city development? In addition, how have modern citizens been effected by the concept of modernity? The study attempts to probe into the fashion behavior existing in the modern society, and to figure out if the idea of modernity would alter modern people’s life according to the ephemerality and mobility featured in the “modernity” proposed by Baudelaire and Simmel. The study is divided into three chapters︰the first chapter explores the thread of thought of the definition of modernity proposed by Baudelaire in 1863, then identifies the connection between the ephemerality feature and fashion. In the second chapter, following Baudelaire’s idea, we discuss how Simmel regards modernity and identify the relationship between the mobility and fashion. This present study will then elaborate their ideas on modernity and fashion, and explore further how ephemerality and mobility are expressed by individual’s behaviors.