Thesis: We take the wage posting approach with search friction to study three issues in labor market. The first issue concerns the premature quitting of workers. Our framework is suitable for contexts such as disability shock, retirement, maternity leaves etc. Such premature quitting creates turnover risks for firms, hence the firms propose wage profiles to minimize or avoid it. In this issue, the asymmetric information plays an important role. We adopt an approach of mechanism design and consider different timings at which the private information is realized. In a follow-up paper, we propose an age-directed policy by which this inefficiency can be alleviated and study its implication on welfare and aggregate output. In the second issue, we revisit welfare analysis of impact of discrimination on skill choice under a multi-dimensional hiring norm along both productivity-related and -unrelated characteristics. We show how strategic skill investment between favored and discriminated group arise. We compare also two wage determination mechanisms (posted and bargained wage). In the third issue, we consider to which extent can the roles of unemployment benefit and minimum wage correct inefficient allocations arising from firms’ market power. Our context concerns small markets where the workers/firms ratio is not large. The market imperfection comes from the fact that in such a small market firms pay less than competitive level of wages. We proceed from an industrial organizational perspective and suggest focusing on both misallocation of labor and surplus when analyzing the effectiveness of the policy instrument.