As one of the most important and influential urban planning approaches, comprehensive plans can be considered from the different and relevant dimensions and aspects such as the planning system, the content of plans, preparation-approval process, and execution procedure. Hence, this research focuses on the reasons for the inefficiency of comprehensive plans, especially concerning the nature and the substance of this urban planning approach. To this end, in addition to reviewing relevant theories and views, objective instances of the mentioned reasons have been pursued. Therefore, Nowshahr’s Comprehensive Plan was assayed for the ex-post evaluation of plan implementation to assess its efficiency and feasibility in certain aspects, including social and physical dimensions. Following a linear and one-way procedure, inflexibility and lack of realism, single vision and mostly physical solutions, providing too many details based on static and solid frameworks, and the impossibility of accurate predictions are the substantial critical reasons for the inefficiency of urban planning based on comprehensive plans approach. These factors are the challenges that Nowshahr’s Comprehensive Plan also suffers from them identically. The unrealistic approach in determining the city limit, an incorrect policy of using open spaces and increasing density in contradiction with the ecological, touristic, and port nature of the city, failure to realize most of the proposed land uses, and consequently, a severe shortage of service spaces, are among the most apparent objective instances regarding the inefficiency of Nowshahr’s Comprehensive Plan according to the mentioned substantial reasons. Considering the reasons for inefficiency and analysis of the objective instances are an essential strategy that can differentiate the substance of urban plans, especially ones prepared based on the evolution in urban planning and substitution of new approaches, from the old and traditional cases and make them successful more than before.