  We performed systematic measurements of magnetic susceptibility on single
crystals of La$_{1.875}$Ba$_{0.125-x}$Sr$_x$CuO$_4$. The dependence of the
superconducting transition temperature on Sr-concentration demonstrates a
step-like pattern upon doping at {\it x}$\sim$0.08 as the crystal structure
changes from low-temperature tetragonal (LTT) to low-temperature orthorhombic
(LTO) phase at low temperature. Upon cooling, an anomalous upturn in the
susceptibility was observed at the structural phase transition between the
LTT-LTO phases under the magnetic field parallel to {\it c}-axis.
