A study to assess the effectiveness of self-instruction module on knowledge regarding medication errors and its prevention among staff nurses at selected hospital, Kollam. The objectives of the study were to; a) assess the level of knowledge on medication errors and its prevention among staff nurses, b) evaluate the effectiveness of self-instruction module on knowledge of sample, c) find association between pretest knowledge and selected demographic variables. Quantitative research approach was selected with one group pretest-posttest design and convenient sampling technique was used for a sample size of 60. The researcher assessed the knowledge of staff nurses using self-structured questionnaire. After conducting a pretest for the samples, the self-instruction module regarding prevention of medication errors was given. After 5 days post test was conducted using the same tool. The result shows that among the samples10% had good knowledge, 38% had average level and 51% had poor level of knowledge on pretest. Inposttest 45% had excellent level of knowledge, 48.3% had good level of knowledge, 6.6% had poor level of knowledge. It was found that the calculated ‘t’ value is greater than table value and there is significant difference in pretest and posttest knowledge scores among samples. The findings of the study suggest that self-instruction module is an effective intervention in improving knowledge of staff nurses.