The adhesion of silver colloids to a polyimide (PI) substrate was investigated in this study. This issue would be of concern when one tried to print electronic circuits on a flexible PI substrate. To enhance the adhesion between them, the PI surface was first roughened by using the sandblasting technique, and varying degrees of roughness were achieved using various sizes of alumina particles and various air pressures for sandblasting. The adhesion strength of silver colloids to the PI substrate after sintering was then measured according to both ASTM D4541 and ASTM 3359 standards. The results indicated that adhesion strength increased proportionally with surface roughness; however, this enhancement effect was lessened when the roughness was above 880 nm. Sintering temperature was also beneficial in increasing adhesion to the substrate; however, the electrical resistance of the silver colloids after sintering also increased with the degree of roughness. This implied that the packing of silver colloids was also affected by surface roughness. Consequently, values of surface roughness were optimal when adhesion strength and electrical resistivity were balanced.