This study was carried out on three cotton groups included Egyp­tian ELS and LS cotton varieties besides some Upland cottons aiming to study the effect of specimen size on Fibrograph and HVI length meas­urements as well as on Stelometer and HVI strergth measurements. The effect of test specimen size on the precision degree (P.D %) of test re-suits was also studied. The results showed that Fibrograph, HVI and Ste­lometer tests should be carried out within a specific range of test speci­men size to get correct values with high degrees of accuracy. The relationship between test specimen size and each of length and strength measurements was also studied.