The purposes of this research were : 1) to develop Web-Based Instruction (WBI) on Official Letters part of the Word Processing Applications Subject using Demonstration Technique; 2) to evaluate if the WBI meets Meguigans efficiency criterion; 3) to compare learning achievements of learners before and after studying by the developed WBI; and 4) to evaluate students’ performance after studying the developed WBI. The sampling group recruited with purposive sampling method were 20 second-year vocational certificate students who registered in the second semester of academic year 2012 majors in Business Computer at Bunharn-Jamsai Polytechnic College Suphanburi province. Applied research tools included one unit of WBI’s evaluation form pre-test and post-test learning achievement test and WBI post-test Students’ performance assessment form. Applied statistic tools included Meguigans efficiency value t-test mean and standard deviation (S.D.) Research findings showed that 1) the efficiency of developed WBI was 2.26 which higher than the Meguigans’s standard value at 1.00. 2) post-test learning achievement was statistically significant higher than pretest at .05 3) students’ post-test performance was at good level. We therefore could conclude that the developed WBI can be utilized as teaching material for students with different computer background.