[en] Compared with a circular tube or/and a rod bundle, the dimensional deviation from the nominal size of a rectangular channel, formed by two plate-type elements, has bigger impact on fluid flow area, particularly, the deviation of the narrow side size. The size impact of narrow side on thermal hydraulic behaviors of rectangular channel (pressure drop and CHF) has been evaluated quantitatively, as a given fuel element as reference. The results reveal that the significant impact on pressure drop will be taken if boiling is caused due to the size deviation. The flow rate of the channel will reduce about 11% as single phase flow replaced by two phase flow, even if quality is 1%. However, for the signal phase flow, 0.1 mm deviation in narrow side decreases the flow rate 3.8%, only. The influence on CHF relates to CHF correlation. For the CHF correlation mentioned, the influence can be negligible if the factor of mass flow rate is considered only