An accurate estimate of crop water requirement is of vital importance in planning, design and operation of irrigation and water resources system. Relationships between the evapo-transpiration of a preselected crop (the reference crop), which is referred to as reference evapo-transpiration (ET0), and other crops are established by multiplying ET0 by crop coefficients. The Penman-Monteith (P-M) equation with its new definition of ET0 is recommended by FAO as the standard method of crop water requirement calculation. Using 54 years climatic data, the ET0 for different regions of Bangladesh were calculated with FAO CROPWAT software. The implication of the ET0 values in crop planning are discussed.