Abstract: The experimental results obtained in scaled thermo-hydraulic test facilitiescan provide basic data and correlations for design, operation and safety evaluationof PWR system, and verify the corresponding models used in computer codes. Ascale test facility may induce modeling distortion due to improper modeling methods,incolmplete design considerations, technological difficulties and some otherreasons. Thus, the accuracy of the experimental results may be affected. However,for a certain test facility used for some special experiment projects its modelingdistortion can not only be minimized but also predicted and modified quantitatively. In order to reduce the distortion, it is neccessary to do more effectiveresearch into the simulating methods. It is a important task to carry out somerelevant experiments for verifying if the experimental results and the codes areaffected by the distortion of the test facility and for determining how toeliminate this effect. The simulating methods in common use, the modelingproblems existing in large test facilities in the world and the effects of modelingdistortion on computer codes are briefly discribed in this paper. An approach ofhow to avoid and/or reduce the distortion is proposed.