The study was conducted to investigate the effects of water and nitrogen on growth and to identify root traits that improve crop water and nitrogen uptake of wheat and barley genotypes at the Faculty of Agriculture Research Station, University of Jordan, Amman, during 2007. Two barley genotypes (Hordeum vulgare L., Vars Rum and ACSAD 176) and two wheat genotypes (Triticum turgidum L. var. durum, vars. Hourani and Om Qaise were subjected to two water treatments (rain-fed and rain-fed plus supplementary irrigation) and three nitrogen levels of 0, 50 and 100 kg N ha-1. Barley produced higher above ground dry matter and Green Area Index (GAI) and leaf area ratio than wheat at anthesis stage. Root Length Density (RLD) was affected by genotype and nitrogen at depths from 0-80 cm. The effect of irrigation on root characteristics was only at depths below 20 cm where rainfed treatments produced higher RLD and root weight as compared to irrigation treatments. Results also showed an exponential decrease in RLD of various genotypes with soil depth. Barley vars had higher root weight and RLD than wheat vars especially at soil depths of 0-40 cm. Positive relationship between RLD and water and nitrogen used by wheat and barley plants was observed. Root water capture rate was higher for barley under irrigation by 4.5 folds as compared to rain-fed. On the other hand, stronger association between RLD and nitrogen uptake were observed for wheat genotypes under both rainfed and under irrigation as compared to barley.