To guide an assembly sequence, instructions are often presented on paper or digital manuals containing photos, diagrams, and texts. Recently, augmented reality (AR) has gained increasing attention as a new way to provide user support and assistance for guiding assembly. To date, although comprehensive studies have compared the effectiveness of augmented reality instructions on Head-Mounted Displays and projectors, only a few investigate the AR instructions on mobile devices such as smartphones and tablets. In this paper, we aim to close this gap by comparing mobile AR instructions to paper-based instructions using a Lego assembly task. Two groups of participants were asked to either use AR instructions or paper-based instructions. Their task effectiveness, efficiency, cognitive load, and motivation were measured. Results indicate that participants who used mobile AR instructions made fewer errors compared to participants who used paper instructions. Furthermore, mobile AR instructions do not hinder task efficiency, cognitive load, and motivation. This study adds empirical evidence to the usability of mobile AR instructions and may help instructional designers gain insights about differences in mobile AR instructions and paper instructions.