This study is conducted to assessment the performance of subsurface drainage system in the 2100 ha; located in El-Bostan region, El Behira governorate Egypt (30^o 47' 00'' and 30^o 51' 00'' N, 30^o 23' 00" and 30^o 25' 00" E). Three sub main drains, El-Amlak No. 7, El-Amlak No. 8 and El-Amlak No. 9 served the studied area. 105 piezometers were installed in selected sites of studied area. Twenty one sets of PVC piezometers were installed perpendicular on the both sides of sub main drain lines at depth of 120 cm from soil surface. Each set was consisted of 5 piezometers were installed at 0.4, 2.5, 5, 15 and 30 m away from the lateral drain. The drainage assessment parameters revealed that, the average drainage intensity factor values were 0.029, 0.018 and 0.030 day^-1 for sub main drains No. 7, 8 and 9 respectively. The average values of days after cessation of recharge (t_A) values in piezometers at midway between lateral drains for sub main drains No. 7, 8 and 9 were 17.20, 23.69 and 13.61 days, respectively. The average value of water table draw down in piezometers sites at midway between drains represents the tile drainage system (sub main drains) at studied area was low (2.33 cm/day), this low value of water table draw down continuously caused the raising of height water table above the drains. Moreover, the average value of water table height above the drains level at midway between drains before the next irrigation for the tile drainage system in the studied area (77.13 cm) is required 33 days as well as 15 days irrigation interval to below the drains level. Thus, the tile drainage system in this area is very poor. The average value of head loss fraction for the tile drainage system in studied area was 1.340. The entrance resistance average of the tile drainage system in studied area was 7.267 day/m. The values of head loss fraction and entrance resistance revealed that the drainage system performance very poor. The obtained value of depleted fraction (DF) was ranging between 0.51 and 0.57 (table 5) which is considered normal for arid region. The critical value of DF = 0.55 implies that if ETa at any month is less than 0.55 (Pe + Vc), a portion of this available water goes into storage, causing the water table to rise under inefficient drainage system.