Background: Unhealthy lifestyle and lack of readiness to modify are some of the most important factors in the development of chronic diseases such as cardiovascular diseases (CVDs). Objectives: To investigate the correlation of heart knowledge, perceived heart risk factors (PHRFs), and risk perception of heart disease with readiness for lifestyle modification in companions of patients with cardiovascular diseases. Methods: In this cross-sectional study, 200 companions of patients with CVDs at Imam Ali Hospital in Kermanshah city were selected by convenience sampling. The data were collected using the heart disease knowledge questionnaire (HDKQ), perception of risk of heart disease scale (PRHDS), perceived heart risk factor scale (PHRFS), and readiness for lifestyle modification questionnaire. Data were analyzed using the Pearson correlation coefficient and linear regression analyses. Results: The participants had a moderate level of readiness for lifestyle modification, PHRFs, and risk perception; while the level of participants' heart knowledge was weak. Moreover, there was a significant relationship between heart knowledge and readiness for lifestyle modification (P = 0.001); however, there was no significant relationship between the risk perception (P = 0.404) and PHRFs (P = 0.073) with readiness for lifestyle modification. Conclusions: The companions of the patients with CVDs have a moderate level of readiness for lifestyle modification. Considering the existence of a significant relationship between readiness for lifestyle modification and heart knowledge, it can be concluded that a higher level of heart knowledge can play an important role in the increased readiness for lifestyle modification by the family members of the patients with CVDs.