The influence of polymer addition and sand type on the water-reducing,strength and cracking properties of mortars was studied.The test results show that the strength and the water-reducing ratio decrease with decrease of the particle size of the sand.To compare the anti-cracking properties of thin mortar with a thickness of about 5 mm on expandable polystyrene(EPS) foam board,two series specimens prepared with ordinary sand and silica sand were outdoor exposed.The observation showed that a lot of cracks appeared in the mortars without polymer after about 5 months,while only a few cracks appeared in the polymer-modified mortars with low polymer/cement ratio at the edge of the specimen.The mortars prepared with ordinary sand exhibit better crack resistance than that with silica sand.The amount of cracks and length of the specimen prepared with ordinary sand but without polymer is only 50% of that of silica sand specimen.The amount of cracks and length of the specimen prepared with ordinary sand and with polymer is only 1/6 of that of the silica sand specimens.