Each organization not only  requires qualified human resources but also  individuals  who  want  to  do  things outside  of their formal duties to support the organization in achieving  success. This study aims to examine and analyze work engagement as a mediator of authentic personal branding and growth mindset on organizational citizenship behavior at Perpustakaan Nasional Republik Indonesia. This study uses a quantitative approach with a structural equation model using Lisrel 8.72. Using non-probability (purposive sampling) in which employee criteria as government employee and librarian, therefore the sample is numbered, 238 respondents. The findings showed that there is a positive and significant effect: 1) work engagement on organizational citizenship behavior, 2) authentic personal branding on work engagement, 3) growth mindset on work engagement, 4) authentic personal branding on organizational citizenship behavior with work engagement as a mediator, and 5) growth mindset on organizational citizenship behavior with work engagement as mediator.