This study aimed at investigating the effect of training in-service teachers of English in language testing on their acquisition and performance in test construction skills. Fifty male and female in-service teachers of English were randomly chosen from the General Diploma, Faculty of Education, Al-Minia University, Egypt. The present study used a semi-experimental method with a pre-test-posttest, control group design. The experimental group was exposed to the twomonth training program (administered in the second semester of the academic year 2017-2018) and the pre-and-post data collection procedures. Instruments employed in the present study included a training program in test construction skills made of a students’ book and teacher’s guide, a written test in the achievement of (TCS) and a written test of performance in (TCS). Analysis of the data by means of a t-test for data from the both the pre-tests and posttests indicated significantly better post achievement gains. The researcher concluded that the proposed program had a positive effect on the in-service teachers’ acquisition and performance in (TCS). Finally, the study presented educational implications and suggestions for additional research in relation to teacher preparation programs.