Background: Colorectal cancer shows a significant increase in South Korea due to westernization of diet, lack of dietary fiber, drinking and smoking, irregular defecation. There are surgery, chemotherapy, radiation therapy in treatment of colorectal cancer. There may be a medication errors in the process of chemotherapy because of its high toxicity, narrow therapeutic index and the health status of cancer patients. Consequently medication errors can cause increasing the risk of death, prolonging hospital stay and increasing the cost. Among medication errors on medication use process, prescribing errors are of particular concern due to higher risk of serious consequences. It is important for pharmacist to prevent the prescribing errors before reaching patient. Therefore we analyzed the prescriptions of colorectal cancer, classified prescribing errors, suggested guideline to reduce prescribing errors and verified the importance of pharmacist's role in prevention of medication errors activity. Methods: We collected the numbers of prescriptions of colorectal cancer(n=2,373) through anti cancer management program and EMR and analyzed the errors of prescriptions by categories from Oct 1st 2011 to Sep 30th 2012 at Chungbuk National University Hospital. We reviewed the prescriptions as follows - patients' characteristics, the result of test, previous prescriptions, characteristics of antineoplastic agents and patients' allergy, drug sensitivity, adverse events. Prescriptions are classified into inpatient and outpatient and analyzed the errors of prescriptions by categories (dosage form, dose, input, diluents, regimen, product). Results: Total prescription number of inpatient and outpatient of colorectal cancer was 1,193 and 1,180 and that of errors was 107(9%) and 22(1.9%), respectively. In case of errors of categories, the number of errors of dosage form is 69 and 8, errors of dose is 15 and 5, errors of input is 9 and 9 in inpatient and outpatient prescriptions, respectively. Errors of diluents is 8, errors of regimen is 3, errors of product is 3 in only inpatient prescriptions. In case of errors of categories by inpatient department, the number of errors of dosage form is 34 and 35, errors of dose is 7 and 8, errors of input is 6 and 3, errors of diluents is 4 and 4, errors of regimen is 2 and 1, errors of product is 2 and 1 in SG and HO, respectively. In case of outpatient department, the number of errors of dosage form is 8 in HO, errors of dose is 5 in HO, errors of input is 5 and 4 in SG and HO, respectively. Conclusions: The rate of errors of inpatient is higher than that of outpatient. Junior doctors are engaged in prescriptions of inpatient and pharmacist need to pay attention to review all prescriptions. If prescribing errors are discovered, pharmacist should contact the prescriber and correct the errors without delay. The guideline to reduce prescribing errors might be upgrading software of anti cancer management program, education for physicians as well as pharmacists and calling prescriber's attention to preventing recurrence of errors.