This study was performed under greenhouse condition and completely randomized design for wheat and canola at near saturated soil moisture condition to field capacity (seven soil matric suction treatments including 20, 40, 60, 80, 110, 300 and 500 cm with three ) in two consecutive years. The results showed that as the soil moisture content decreases from saturation condition, root respiration rate increases. The range of soil moisture with a maximum root respiration rate in the sandy loam soil was wider in wheat (0.22-0.29 cm3cm-3) than canola (0.24-0.26 cm3cm-3). In sandy loam soil, maximum root respiration rate was higher for wheat (9.81 mol m-2 s-1) than canola (7.62 mol m-2 s-1). While in the clay loam soil, maximum root respiration rate of wheat and canola was approximately equal (10 mol m-2 s-1) at a specified soil moisture content (0.42 cm3 cm-3). Moreover, in the sandy loam soil, the air-filled porosity of 13 and 16%, respectively for wheat and canola were associated with soil moisture content corresponding to maximum root respiration rate. While in the clay loam soil, the air-filled porosity of 17% was for both plants. The results showed that, among the various morphological features of roots, the root volume is the most important feature that controls the respiration rate of the plant roots.