Phat Diem stone cathedral in Ninh Binh province is a unique model of Euro-Asian architecture. Built more than one hundred years ago, this is one of the centers of Catholicism in Northern Vietnam. Especially, within the cathedral's space, one can find a stone chaple built for veneration of Mary known as "Immaculate Heart of Mary". This paper begins with a brief overview of the history of the cathedral. Then it describes and analyzes artistic features of the chapel, with the focus on the Vietnamese representation of Our Lady of Immaculate Conception. Finally, the paper provides some findings on worship activities of Mary.