The spread of Covid-19 in Indonesia has had an impact on the country’s education system. Learning occurs not only in real objects but also in cyberspace, such as through internet intermediaries. This requires the adaptation and sustainability of the teaching-learning process at all levels of guidance. Online learning has been designated as a distance learning management system by the Indonesian government. This is an effective method of keeping a classroom active even when the school is closed in order to prevent the spread of Covid-19. Many institutions have implemented digital source platforms. The simple-to-understand and simple-to-use learning method will delight both students and teachers. In the current era, technology has greatly facilitated the learning process; students and teachers can effectively provide complete assignments and discussion from outside the classroom, especially now that students are going to fight in the pandemic era. The Simaster is one application that has been used in learning. Face-to-face instruction is being replaced by full time online learning from home. Despite this significant shift, the literature on students’ perceptions of the topic continues to remain deafeningly silent. As a result, the current study focuses on the perceptions and challenges to materials used in these online classes of 15 ESP students. The focus of this research is to undergraduate research’s perceptions and challenges while learning ESP with Simaster. The qualitative method was used in this study. To collect the data, interview was used as the primary tool. The interview data is analyzed and see how students perceive and the challenges encountered in ESP by using the Simaster application in pandemic era. The data analysis are data reduction, display data, and the last steps taking conclusion. The findings show that students seemed to have various perceptions of the Simaster application in ESP learning, such as easy access to communication or discussion with teachers and other students. However, when used in assignment activities, this app is very useful because students can easily access and submit assignments without having to meet with the lecturer in person; this saves a significant amount of time. With this simaster, students can attend lectures anywhere and at any time, and lecturers can fulfill their lecture schedules even if they are involved in other activities that cause students to leave class. Because students lack understanding of lecture ESP learning through lectures using this application, Simaster cannot replace face-to-face lecture activities. The students also encountered some challenges