Abstract: The critical heat flux (CHF) performance of fuel rod assembly is the basis of thermal-hydraulic design and safety analysis of power water reactor (PWR), which is very important to the safe operation of the reactor. Compared with the fuel rods, the guide thimble of fuel assembly exists in the form of a cold wall without heating. Guide thimble will cause the increase of boundary layer fluid and decrease the effective coolant flow through the fuel rods, and then may affect the CHF of the rod bundle. This phenomenon is usually called cold wall effect. In this paper, two groups of CHF tests of 5×5 rod bundles were carried out on Large Thermal-hydraulic Test Facility ( LATHY) in Longgang Shenzhen, which are used to study the cold wall effect. These two groups of tests are truncated cosine distribution with axial power peak value of 1.55. The first group of test uses 25 heating rods with diameter of 9.5 mm, the second group uses 24 heating rods with diameter of 9.5 mm and one guide thimble with diameter of 12.45 mm. First of all, comparing the above two sets of CHF test data directly, it is found that the existence of guide thimble will not reduce the average power of rod bundle under consistent boundary conditions, but will lead to the change of the position of burn-out point and reduce the CHF of rod bundle. Then, according to the CHF test data of 58 typical cells grid and all of the 129 CHF test data (including 58 typical cells grid and 71 guide thimble cells grid), the corresponding CHF correlations were developed respectively. The verification results show that the m/p values calculated by the developed two CHF relations have no obvious tendency with local parameters. It is found that the effect of cold wall on CHF is different with local pressure, local mass flow rate and local steam quality, which shows that the decrease of CHF is nonlinear with small change of local pressure, but linear with large change of local mass flow rate and local steam quality. In order to quantify the influence of cold wall effect on CHF, the expression of cold wall effect factor of guide thimble was defined in this paper. Then, applying the CHF correlation developed by using typical cell grid CHF test data is directly to the guide thimble cell grid CHF test data, the cold wall effect factor of guide thimble (about 8%) is obtained.