One key issue for small specimen test techniques is to clarify specimen size effects on test results. In consideration of size effects on determining the ductile-to-brittle transition temperature (DBTT) in Charpy impact testing, a method to evaluate the plastic constraint loss for differently sized Charpy V-notch (CVN) specimens is proposed and applied to a ferritic-martensitic steel, 2WFK, developed by JNC. In the method, a constraint factor, a, that is an index of the plastic constraint is defined as alpha = sigma(*)/sigma(y)(*). Here, sigma(*) is the critical cleavage fracture stress which is a material constant and sigma(y)(*) is the uniaxial yield stress at the DBTT at the strain rate generated in the Charpy impact test. The procedures for evaluating each of sigma(*) and sigma(y)(*) are described and a result of sigma(*) and sigma(y)(*), thus the value of alpha, is presented for different types of miniaturized and full-sized CVN specimens of 2WFK. (C) 2004 Elsevier B.V. All rights reserved.