The deformation behavior of single-crystalline Al0.3CoCrFeNi high entropy alloys (HEA) during tensile test is examined via molecular dynamics (MD) simulation. The impacts of the temperature and strain rate on the tension toughness, Young’s modulus, dislocation propagation, and phase transformation are investigated. The results indicated that phase transformation and dislocation appear in all samples. Rising strain rates result in a growth in the maximum toughness and Young's modulus. In detail, when the strain rate is increased from 108 to 1010 s−1, the maximum stress rises from 11.2 to 12.6 (GPa). Furthermore, at greater strain rates, the percentage of amorphous and hexagon close-packed (HCP) structures increases significantly, resulting in the significant growth of the number of dislocations. On the contrary, the temperature increase results in the reduction of tension toughness and Young's modulus. The maximum stress decreases from 11.6 to 6.98 (GPa) with increasing temperature. Especially recorded amorphization process was due to a significantly increased amorphous content, which is the cause for the decrease in the dislocation and strength in the single-crystalline Al0.3CoCrFeNi HEA samples.