Advance care planning (ACP) and advance directives (ADs) ensure patient autonomy in end-of life care. The number of ADs made and followed in Taiwan is still lacking. This study aimed to determine the factors that influence the willingness to participate in ACP among outpatients in Taiwan. In this study, we conducted a cross-sectional survey based on convenient sampling methods. The questionnaire included questions about participants’ basic sociodemographic information, knowledge of ACP, and awareness of ACP. A total of 198 adults who were outpatients of a family medicine clinic in an affiliated hospital in Taiwan were recruited. The associations between each variable were evaluated using the χ2 test. The adjusted odds ratios (ORs) and 95% confidence intervals (CIs) were calculated using the logistic regression method to examine the influence of each variable on willingness to participate in ACP. Being happy and being a healthcare professional were positively correlated with ACP participation. A lack of ACP knowledge (OR = 0.30 in model A and OR = 0.42 in model C), valuing “Reducing families’ end-of-life decision-making burden” (OR = 2.53 in model B and OR = 2.65 in model C), and a “Belief in a good death” (OR = 4.02 in model B and OR = 4.10 in Model C) were the main factors affecting subjects’ willingness to participate in ACP. Knowing which factors influence willingness to participate in ACP helps in the promotion of ACP. Continuously educating both the general public and healthcare professionals strengthens knowledge about the right to autonomy, about its associated laws, and about the ACP process, and thus, programs should be created to provide this education. Additionally, taking into account the differences between cultures can be helpful.