The main purpose of the this study is to investigate the challenges facing the implementation of CBC in junior secondary schools in Kirinyaga west subcounty ,Kirinyaga county .The objective of the study is to evaluate the readiness of the teachers in the implementation of the new curriculum, understand how inadequacy of infrastructure and teaching learning resources is a challenge in the implementation of the new curriculum and understand the role of the parents cooperation in the implementation of the CBC. The study is guided by the following research questions; how are the teachers prepared for the implementation of CBC, what are the infrastructural and teaching learning resources used during the implementation of the new curriculum and how are the parents cooperating in the implementation of the new curriculum. The study used vygotsky theory . The study used descriptive research design The target population was both the teachers and headteachers.The data was collected using simple random sampling techniques .Questionnaires were used to collect information .Qualitative data was analysed as per the objectives of the study .Finding showed that the schools have inadequate infrastructures for facilitating learning .Teachers have inadequate knowledge and the parents do not fully cooperate to promote effectiveness of the new curriculum ..The study concluded that teachers are not fully prepared for the new curriculum, the schools have inadequate infrastructure and teaching learning resources and the parents are not fully cooperating with the teachers .The study recommended the government through the ministry of education to provide adequate teaching learning resources and infrastructure ,employ more teachers ,and make arrangements for continuous action research to enhance improvements in the new curriculum