Error Analysis (EA) in Second Language Acquisition (SLA) is one of the interesting studies in linguistics, especially when the errors being analyzed are made by the Indonesian people. English is a foreign language for Indonesian but this language, in STIE Perbanas Surabaya, is considered one of the components in the administration staff’s performance appraisal (PA). This paper attempts to explore the possible causes of errors of English written by the staff of STIE Perbanas Surabaya. The subjects being analyzed are the written English taken from the test results of English done recently. The data were taken from 20 sheets of test results concerning the two types of written test. The first was the guided writing test and the second the free writing test. The data were analyzed using the theory of Error Analysis (EA) by firstly, classifying into the levels of sentence construction: phrases/ word formation and sentences. It can be generalized that most of the staff were influenced by their first language (L1) in both the morphological construction and the sentence levels. It is advisable for the institution to let them exposed to English in the staff in daily lives so that they can acquire English.