A method for manufacturing a ceramic substrate is characterized in using a preformed trench, a patterned protective layer and a sand blasting process to manufacture a cavity in a ceramic substrate and control the cavity size and shape of the ceramic substrate. The ceramic substrate is collocated with a base substrate to form a package substrate for packaging a semiconductor chip. The manufacturing method set forth above can lower the manufacturing cost and raise the accuracy of the size and shape of the cavity of the ceramic substrate. The abovementioned method can reduce the fabrication cost and increase the precision of the shape and size of a ceramic substrate.