It is aimed to investigate the effect of portfolios and technology enhanced teaching on developing writing skills and their success among 78 Turkish undergraduate learners in ESP classes. This study was carried out on three groups – two experimental and one control groups- each of which consists of 26 students. The learners were freshmen at the department of Sea and Marine Management Program at Gerze Vocational School of Higher Education, Sinop University. Before the treatment, a pre-test was administered to all groups in order to investigate the writing success and sub-skills of focus, elaboration, organization, conventions and vocabulary of learners of ESP. Throughout the study one group was taught through portfolio assessment technique and the other group was taught through the technology-based method while the control group applied conventional method. An Independent samples t-test was applied in order to see whether there was a statistically significant difference or not between the experimental and control groups. Additionally, a Paired Samples t-test was used so as to compare differences within each group. In order to investigate the relation between the gender and the writing success of the students Pearson Correlation Test was applied. The results obtained from the post-test demonstrated that there was a statistically significant difference between the two experimental groups and control group in terms of the type of treatment, which signifies that portfolios had a statistically significant effect on promoting writing performance and its sub-skills in ESP classes. After the treatment, the students in experimental groups were also administered an attitude questionnaire to elicit their thoughts about the effectiveness of using portfolios on augmenting writing abilities in ESP classes. The data collected from the attitude questionnaire was analysed through descriptive statistics including the frequencies, percentages, means, and standard deviations etc. The results showed that students have positive attitudes towards using portfolios on improving their writing skills in ESP classes.