  We are now operating the 500 m2 Yangbajing air-shower core (YAC-II) array
near the center of the Tibet air-shower array (Tibet-III) to observe cosmic-ray
chemical composition at the knee energy region since February 2011. The first
step of YAC, called YAC-I, containing 16 detector units, was operated from May,
2009 to February, 2010. In this paper, we used the YAC-I and Tibet-III
coincident data set obtained from May, 2009 through January, 2010 to present
the electromagnetic spectrum of air shower cores at around 1015 eV energy
region. The effective live time is calculated as 100.5 days. We would like to
report the comparison of our experimental data with MC model prediction in this
paper.
