Korean IT industry has noticed innovative changes emerging along with the increased popularity of smartphones. Increase of smartphone user has extended the smartphone business arena from simple and personal applications and content to professional software for the purpose of working in- and out of the office. In this regard, developing their services to be mobile-friendly would be important business strategies for Web business companies. The mobile data traffic in Korea had been 11.2 times increased from January 2010 to January 2011 and the average traffic per user in Korea is much higher than other countries. Usage of smartphones also has been steadily increased with the diffusion of smartphones. This may indicate that the dependency level on mobile portal service in Korea would be higher and more important than in other countries. This study analyzes the influences of UI simplicity and UI consistency on user perceptions of mobile portal services using PLS methodology. Simplicity shows a greater effect on usability and credibility than does consistency although consistency also shows a significant effect. In this regard, developing mobile Web services to be simple by following the selection and concentration strategy can be an effective strategic approach. Credibility shows a greater and direct effect on user satisfaction in this study than simplicity. But it does not mean that the perceived credibility should be treated simply as more important to user satisfaction than usability. Credibility of mobile Web services would be concreted more when the perceived usability would be developed with proper UI simplicity and consistency following the suggestion of Mann and Sahni. Also satisfaction significantly turns out to mediate the effect of credibility on loyalty. This study contributes as an earlier study on how and what the mobile Web service providers should design and provide their services.