In this study an output-only system identification technique for civil structures under ambient vibrations is carried out, mainly focused on the Subspace System Identification (SSI) based algorithms. With the aim of finding accurate and true modal parameters, a stabilization diagram is constructed by plotting the identified poles of the system with increasing the size of data matrix. Comparative study between different approach, with and without Singular Spectrum Analysis to preprocess the data, on determining the model order and selecting the true system poles is examined in this study. Identification task of the real large scale structure: Guangzhou New TV Tower (GNTVT), a benchmark problem for structural health monitoring of high-rise slender structures is carried out, for which the capacity of SSI-based algorithm is demonstrated.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.