Cheju island is situated at about 60km south of southern most tip of the Korean peninsula. Its shape is an ellips with size of 80*40$ extrm{km}^2$, and it is characterized by a symmetrical form whose peak is Mt. Halla(1950m). Major chemical composition of the rock samples from Manjang gul cave is determined by XRF using the fundamental parameter method consisting of fully auto quantitative analysis, semi quantitative analysis (order estimation) and group quantitative analysis. Judging from chemical composition (SiO$_2$ = 53.07Wt％, Fe$_2$O$_3$ = 11.34Wt％, MgO=6.48Wt％, Na$_2$O=3.07Wt％, $K_2$O=1.05Wt％), this rocks from the Manjang gul cave are also determined. The discrepancy of K-Ar ages is found. They are 0.03 Ma and 0.42Ma, respectively. This paper describes some problems experienced in dating young volcanic rocks and then discusses chemical composition, X-ray fluorescence analysis and the age of the formation of a lava tunnel such as Manjang gul cave in cheju Island.