This study will compare the efficacy of a Chatbot-delivered stage of changes (SOC)-tailored online intervention (intervention group) versus a Chatbot-delivered non-SOC-tailored online intervention in increasing the prevalence of meeting WHO recommended physical activity level six months after completion of the intervention among inactive community-dwelling individuals aged ≥65 yeas. In the intervention group, the Chatbot will assess participants' physical activity level and SOC regarding PA and automatically deliver interventions tailor to their SOC every week for 12 weeks. In the control group, the Chatbot will not assess participants' SOC. It will assess participants' physical activity level and provides a standard intervention covering general information about physical activity for older adults, which is not tailoring to participants'SOC every week for 12 weeks. All participants will be followed up after completion of the interventions (T1) and six months after T1 (T2).

Inclusion Criteria:

• Aged ≥65 years,
• Having a Hong Kong ID,
• Chinese speaking,
• Not meet the WHO recommended level of MVPA
• Willing to be followed by telephone at 3 and 9 months,
• Having a smartphone
• Able to send and read text/voice message via smartphone.
• Willing to wear an accelerometer for a week at T0, T1 and T2.

Exclusion Criteria:

• Blindness or deafness,
• Having been diagnosed with major psychiatric illness (schizophrenia or bipolar disorder) or dementia,
• Score ≥1 in the Chinese version of the Physical Activity Readiness Questionnaire (PAR-Q)
• Score ≤16 in the validated telephone version of the Cantonese Mini-mental State Examination (T-CMMSE)