Dynamic assessment integrates the teaching and testing process. Several studies have been conducted to find out the role of dynamic assessment in enhancing students' writing skills (Alemi, 2015; Amiri & Saberi, 2016; Sadek, 2015; Shresta & Coffin, 2012). However, studies on its implementation in English for Specific Purposes (ESP) settings are limited. This study aimed to find out the effectiveness of dynamic assessment in improving students' writing skills, especially the genre of descriptive text in an ESP class. A mixed-method design was used. Seventy students in a Library Technician Program at a public university in Indonesia participated in the study. The data were gathered through pretests and posttests. An analysis of T-test scores showed that they were statistically significant, suggesting that the dynamic assessments had a positive influence on students' abilities in composing texts. Following these tests, an open-ended questionnaire was administered to obtain students' opinions. The responses from the questionnaire revealed that most students had positive views toward dynamic assessments as it helped them assess their writing skills, find specific vocabulary, and reduce anxiety.