The effect of grain size on the mechanical properties in ultra-high pure aluminum had been investigated as a function of strain rate. Specimens with average grain diameter sizes of 243, 678 and 1070 m were compressed and elongated at quasi-static and high strain rates by a computer controlled servo-hydraulic testing machine and a Split Hopkinson Pressure (Tension) Bar (SHPB and SHTB). The mechanical properties were found to vary significantly with grain size, and strain rate. The relationship between flow stress and grain size can be expressed by a Hall - Petch relation with the different slope for both compressive tests and tensile tests. The influence of strain rate on the slope of the Hall - Petch relation is such that in compression, the slope does not change much, but in tension, there is an increase in the slope value. The strain hardening rate was seen to increase with increasing strain rate. The strain rate dependence of flow stress is obvious, and is seen to be more significant for the smallest grain size specimens. The 3D fractographs illustrated that the numbers of the dimples decrease with the increase of the grain size.