Objective To find out the best time and investigate the indications for conversion to horacotomy in completely thoracoscopic lobectomy. Methods Between Sep. 2006 and Feb. 2009, 172 patients including 88 male and 84 female with the median age of 58.9 years, underwent completely thoracoscopic lobectomy. Postoperative pathology showed that there were 133 cases of primary lung cancer, 7 cases of lung cancer metastasis and other malignant tumors, and 32 cases of benign diseases. Among them, 46 patients had the tumor on the right upper lobe (RUL), 23 on the right middle lobe (RML), 31 on the right lower lobe (RLL), 36 on the left upper lobe (LUL) and 36 on the left lower lobe (LLL). Three incisions were made in all operations. The procedures of systematic lymphadenectomy and anatomic lobectomy were similar with routine thoracotomy. If there was mediastinal lymph node adhesion, metastasis or bleeding, the incision would be extended to 12-15 cm and the surgery would be converted to thoracotomy. According to whether the maximum tumor dimension was above 5 cm or under 3 cm, the patients were divided into two groups. At the same time, we also divided the patients into two groups based on whether thoracotomy was performed. The data of both two groups were compared respectively. Results All surgeries were carried out safely with no serious complications or perioperative deaths. The average surgical duration was 185 minutes, and the average blood loss was 213 ml. Thirteen operations were converted to thoracotomy with a conversion rate of 7.6%. Among them, 9 were interfered by lymph nodes and bleeding happened in 4 operations. Lobectomy was performed on 12 patients and pneumonectomy was performed on 1 patient after thoracotomy. For the 16 cases of tumor with its dimension larger than 5 cm, the average operation time was 187 minutes and the average blood loss was 203.8 ml, while for the 98 cases of tumor with its dimension smaller than 3 cm, the average operation time was 202 minutes and the average blood loss was 231.3 ml. The difference between these two groups was not statistically significant. Among the 13 cases of conversion to thoracotomy, the mean age of the patients was 68.7 years old and the average tumor dimension was 23.8 mm. For the 159 cases without thoracotomy, the average age was 59.3 years old and the tumor dimension averaged 27.8 mm. There was a significant difference between them (P=0.016). Conclusion Interference by lymph nodes and bleeding are the most important causes of conversion to thoracotomy in completely thoracoscopic lobectomy while size of tumor, fused fissure or plural adhesions can be always managed thoracoscopically.