Forward quantification of uncertainties in code responses require knowledge of input model parameter uncertainties. Nuclear thermal-hydraulics codes such as RELAP5 and TRACE do not provide any information on physical model parameter uncertainties. A framework was developed to quantify input model parameter uncertainties using Maximum Likelihood Estimate (MLE) and Expectation- Maximization (E-M) algorithm for physical models using relevant experimental data. The objective of the present work is to perform the sensitivity analysis of the code input (physical model) parameters in TRACE and calculate their uncertainties using an MLE algorithm, with a particular focus on the subcooled boiling model. In this paper, the OECD/NEA BWR full-size fine-mesh bundle test (BFBT) data will be used to quantify selected physical model uncertainty of the TRACE code. The BFBT is based on a multi-rod assembly with measured data available for single or two-phase pressure drop, axial and radial void fraction distributions, and critical power for a wide range of systems conditions. In this study, the steady-state cross-sectional averaged void fraction distribution from BFBT experiments is used as the input for MLE algorithm, and selected physical model Probability Distribution Function (PDF) is the desired output quantity.