Spherical $MgB_2$ powders was synthesized with the ultrasonic spray pyrolysis(USP) process using aqueous solutions of boron and magnesium ion. The properties of synthesized $MgB_2$ powder were characterized by XRD, SEM and EDS. A small amount of MgO was detected as the secondary phase out of the synthesized powder and the ratios of $MgB_2$ to MgO increased with increasing furnace temperature. The particle size and morphology of $MgB_2$ powder were investigated with varying molar concentration of the boron and magnesium solution and furnace temperature between $600^{circ}C$ and $1000^{circ}C$ in $Ar/H_2$. The average particle size of $MgB_2$ showed narrow distribution ranging from 300nm to 400nm. The morphology of particles exhibited mostly spherical shapes and uniform distribution.