Fibrous cap thickness (Cap_thick) is often considered as diagnostic of the degree of plaque instability. Necrotic core area (Core_area) and the arterial remodeling index (Remod_index), on the other hand, are difficult to use as clinical morphological indices: literature data show a wide dispersion of Core_area thresholds above which plaque becomes unstable. Although histopathology shows a strong correlation between Core_area and Remod_index, it remains unclear how these interact and affect peak cap stress (Cap_stress) - a known predictor of rupture. The aim of this study was to investigate the change in plaque vulnerability as a function of necrotic core size and plaque morphology. Cap_stress value was calculated on 5,500 idealized atherosclerotic vessel models, which had the original feature of mimicking the positive arterial remodeling process described by Glagov. Twenty four non-ruptured plaques acquired by IVUS on patients were used to test the performance of the associated idealized morphological models. Taking advantage of the extensive simulations, we investigated the effects of anatomical plaque features on Cap_stress. It was found that: (i) at the early stages of positive remodeling, lesions were more prone to rupture, which could explain the progression and growth of clinically silent plaques; and (ii) in addition to cap thickness, necrotic core thickness - rather than area - was critical in determining plaque stability. This study demonstrates that plaque instability is to be viewed not as a consequence of fibrous cap thickness alone, but rather as a combination of cap thickness, necrotic core thickness and the arterial remodeling index.