This study attempted to examine the effects of collaborative writing on EFL students' paragraph level writing performance focusing on the two aspects of writing: content and coherence. Two batches of Grade 11 students at Felegebirahn Secondary School in Amhara Region, Ethiopia were selected for the study group based on the mean scores of the paragraph writing performance test given before the intervention. These two batches of students were randomly assigned into two groups: experimental (n=44) and control (n=44) using lottery system to conduct the study. The students in the experimental group were made to practice paragraph level writing tasks collaboratively; while the students in the control group were made to practice the same writing tasks individually for 12 weeks. The main instrument used to collect the data was paragraph writing test. Students' questionnaire and semi-structured interview were also used to gather data regarding students' attitude towards using collaborative writing in EFL classes. The paragraph writing test results were used to examine and compare students' paragraph writing performance before and after the intervention. T-test was employed to analyze and interpret if the paragraph writing performance tests mean differences with-in the groups and between groups were statistically significant or not. The findings revealed that the students who practiced the writing tasks or activities collaboratively have brought more significant improvements on the content and coherence of the paragraphs they produced after the training than students who practiced the writing tasks individually. It was also noted that the students in the experimental group had exhibited positive attitude towards collaborative writing. This was confirmed by majority of the respondents from the attitude questionnaire and interview data that students finally enjoyed and were motivated to write in English after their engagement in collaborative writing. It was concluded that practicing writing tasks collaboratively in EFL writing classes can improve students' performance to incorporate relevant and coherent ideas or sentences while students write paragraphs in English. Therefore, it was recommended that using collaborative writing in EFL writing lessons must continue and be adopted on wider scale.