Hot tensile tests were carried out to investigatehot ductility behavior and critical temperature range(AT) between zero strength temperature (ZST) and zeroductility temperature (ZDT) of tool steel grades AISIL3 and 01. The test specimens solidified from meltingat cooling rate of 0.5 degree C/s and 3 degree C/s tothe test temperature in the range between liquidus and900 degree C. The specimens were then strained tofailure at strain rate of 2x10('-3)/s and 2x10('-2)/s.Quenching test was carried out in order to investigatethe microstructure occurred during hot tensile test.The microstructure that appeared in quenched specimenwas then identified with SEM-EDX. It was found that the critical temperature rangeof the steels solidified at the slow or the fastcooling rate, widened with increasing strain rate. Thespecimens which solidified at slow cooling rate whentensile strained to failure at low or high strain rate,had wider critical temperature range than the specimensthat solidified at fast cooling rate. For steel gradeAISI L3, tensile test at high and low strain rate(after solidified at the fast cooling rate) revealedthe hot ductility of 35-65% RA between 900-1250 degreeC. Whereas, the hot ductility of the samples whichsolidified at slow cooling rate and pulled at slowstrain rate were markedly higher than those pulled athigher strain rate between 900-1200 degree C.Precipitation of manganese sulfide in theinterdendritic region resulted in deterioration of hotductility of steel grade AISI L3. For steel grade AISI01, tensile tests of specimens solidified between900-1100 degree C at slow cooling rate and low strainrate yielded higher hot ductility than those specimenspulled at high strain rate. The hot ductility ofsamples at fast cooling rate and low strain rate showedno difference to those samples of hot tested at highstrain rate. Nevertheless, slow cooling rate affectedhot ductility more than fast cooling rate for tensiletests at both high and low strains rate. Carbideprecipitates of vanadium, tungsten and chromium alongaustenite grain boundaries influenced the ductilityloss of steel grade AISI 01.