A series of polycrystal bulk samples of (La1-yTby)2/3Sr1/3MnO3(x=0,0.05,0.15,0.20,0.25,0.33,0.4,0.5,0.6,1.0) have been prepared. Their microstructrues, magnetization M, resistance R as well as magnetoresistance (MR) have been systemically studied in the temperature range 10K≤T≤420K. The ferromagnetic transition temperature of the samples TC decrease with the increase y. For each sample, R have peak values at a temperature close to TC. The peak values of R increase rapidly with increasing y. At the sample (y=0.40), a colossal magnetoresistance up to 900% has been observed on the temperature 170K.